Some of the most common words in the English language are also the most confusing when it comes to grammar. Words like of, is, to, that, a, for, and in appear in virtually every sentence — yet most people couldn't tell you what part of speech they belong to if their grade depended on it.
If you've ever stared at a grammar worksheet wondering what part of speech is "of"? or is "that" a conjunction or a pronoun?, you're in the right place. This guide gives you straight answers with real examples, so these tiny words never trip you up again.
What Part of Speech Is "Of"?
"Of" is a preposition — and not just any preposition. It's the most frequently used preposition in the English language, appearing in everything from casual conversation to academic writing.
Prepositions are words that show the relationship between a noun (or pronoun) and another element in the sentence. They typically express relationships of place, direction, time, or — in the case of "of" — possession, origin, composition, or association.
Here's how "of" works in different contexts:
- Possession: the cover of the book (the book's cover)
- Composition/material: a cup of coffee, a wall of stone
- Origin: the city of Rome, a native of France
- Quantity: a dozen of them, most of the students
- Association: the University of Michigan, the smell of rain
In every case, "of" links a noun to something else and clarifies their relationship. That's the job of a preposition — and "of" does it constantly.
Why Does "Of" Confuse People?
"Of" is so short and so common that it feels invisible. Unlike longer prepositions such as "beneath" or "alongside," "of" doesn't feel like it's doing much. But remove it from a sentence and the whole thing collapses: "the smell rain" or "a cup coffee" make no sense. "Of" is doing heavy lifting, quietly.
One common mistake: writers sometimes swap "of" for "have" in speech-influenced writing. For example, "I should of gone" is incorrect — the correct form is "I should have gone." The spoken contraction "should've" sounds like "should of," which causes the confusion.
What Part of Speech Is "Is"?
"Is" is a verb — specifically, it is the third-person singular present tense form of the verb to be.
Verbs are words that express actions, states, or occurrences. "To be" is a linking verb (also called a copula), which connects the subject of a sentence to a description or identifier rather than expressing a physical action.
- She is a doctor. (links subject to identifier)
- The sky is blue. (links subject to adjective)
- It is raining. (auxiliary verb helping the main verb)
"Is" can also serve as an auxiliary (helping) verb when forming continuous tenses: He is running. She is working. Whether it's linking or helping, "is" is always functioning as a verb.
What Part of Speech Is "To"?
This one has a twist: "to" can be either a preposition or part of an infinitive — and the distinction actually matters.
"To" as a Preposition
When "to" shows direction, destination, or relationship, it's a preposition:
- She walked to the store.
- He gave the gift to her.
- They moved to Canada.
"To" as Part of an Infinitive
When "to" appears directly before a verb in its base form, it creates an infinitive. In this role, it's sometimes called an infinitive marker rather than a true preposition:
- To run is her passion.
- She wants to learn French.
- He tried to apologize.
A quick test: if you can replace "to" with "toward" and the sentence still makes sense, it's a preposition. If not, it's an infinitive marker.
What Part of Speech Is "That"?
"That" is one of the most grammatically versatile words in English. Depending on how it's used, "that" can be a pronoun, a determiner (adjective), a conjunction, or a relative pronoun.
- Demonstrative pronoun: That is my car. (replaces a noun)
- Determiner: That car is mine. (modifies a noun)
- Subordinating conjunction: I know that she is right. (introduces a clause)
- Relative pronoun: The book that I read was fascinating. (introduces a relative clause)
Context is everything with "that." Look at what job the word is doing in the sentence — is it standing in for a noun, modifying one, or connecting clauses? That answer tells you which part of speech it is.
What Is the Part of Speech of "A"?
"A" is an article, which is a subcategory of determiners (and sometimes classified under adjectives in traditional grammar).
English has three articles: a, an, and the. The articles a and an are indefinite articles — they refer to a non-specific noun. The is the definite article, referring to something specific.
- I saw a dog. (any dog, not specific)
- I saw the dog. (a specific dog we both know about)
"A" is used before consonant sounds; "an" is used before vowel sounds: a university (sounds like "yoo"), an umbrella (sounds like "uh").
What Part of Speech Is "For"?
Like "to," "for" is primarily a preposition, but it can also function as a coordinating conjunction in formal or literary writing.
"For" as a Preposition
- This gift is for you. (beneficiary)
- She studied for three hours. (duration)
- He left for Paris. (destination)
- We use it for cooking. (purpose)
"For" as a Coordinating Conjunction
"For" is one of the seven coordinating conjunctions in the FANBOYS group (For, And, Nor, But, Or, Yet, So). In this role, it means "because" and joins two independent clauses:
- She stayed inside, for it was raining heavily.
This usage is relatively formal and more common in older or literary texts.
What Part of Speech Is "In"?
"In" is most commonly a preposition, though it can also function as an adverb or, in some cases, an adjective.
"In" as a Preposition
- Location: She is in the kitchen.
- Time: We'll meet in an hour.
- State or condition: He is in trouble.
"In" as an Adverb
- Come in! (modifies the verb "come," no object follows)
- She walked in.
The key difference: when "in" has a noun object after it, it's a preposition. When it stands alone modifying a verb, it's an adverb.
A Quick-Reference Summary
- Of → Preposition (shows possession, composition, origin, association)
- Is → Verb (linking or auxiliary form of "to be")
- To → Preposition or infinitive marker (depends on context)
- That → Pronoun, determiner, conjunction, or relative pronoun (highly context-dependent)
- A → Article / Determiner (indefinite article)
- For → Preposition (usually) or coordinating conjunction (formal/literary)
- In → Preposition (usually) or adverb (when standing alone)
How to Stop Making Grammar Mistakes With These Words
Understanding parts of speech isn't just an academic exercise — it directly improves your writing. When you know that "of" is a preposition, you'll stop writing "should of". When you understand that "that" can be a conjunction, you'll use it more confidently in complex sentences.
That said, even experienced writers make mistakes with these small, deceptively simple words. If you want a safety net, the WriteGenius Grammar Checker can instantly flag errors involving prepositions, verb forms, and other grammatical slip-ups — it's especially useful for catching the kind of subtle mistakes that spell-checkers miss.
And if you're ever unsure whether a sentence you've written actually sounds right, try rephrasing it with the WriteGenius Paraphraser. It can offer alternative constructions that make the grammar clearer without changing your meaning.
The Bottom Line
English's most common words — of, is, to, that, a, for, in — are small, but they carry enormous grammatical weight. Most of them are prepositions or function words that glue sentences together. A few, like "that" and "to," are chameleons that change roles depending on context.
The best way to identify any word's part of speech is to look at how it functions in its specific sentence — not just what the word is, but what job it's doing. Ask: Is it connecting? Describing? Standing in for a noun? Showing a relationship? The answer will point you straight to the right grammatical category.
Grammar doesn't have to be intimidating. Once you see the logic behind these tiny words, the whole system starts to click.