Are all words that end in ar er and ir infinitives?

No, not all words ending in "ar," "er," or "ir" are infinitives.

Here's why:

* Infinitives are the base form of a verb, usually preceded by "to" (e.g., "to walk", "to run", "to sing").

* While many infinitives in English do end in "-ar", "-er", or "-ir", these suffixes are also common in other parts of speech, like:

* Nouns: "guitar", "slicer", "barrier"

* Adjectives: "popular", "clear", "superior"

* Adverbs: "further", "earlier", "earlier"

To determine if a word is an infinitive, consider:

1. The word's function in the sentence: Is it acting as a verb, or is it another part of speech?

2. The presence of "to": Is the word preceded by "to"?

Examples:

* Infinitives: "to repair," "to prefer," "to stir"

* Not Infinitives: "guitar," "clearer," "barrier"