-ть

The infinitive is the citation form you will find in any dictionary. In Russian, the vast majority of verbs end in the suffix -ть.

1. Usage:
The infinitive is used in several key ways:

• After Modal Words: When you want to say you 'want' or 'can' do something (e.g., Я хочу спать - I want to sleep). Unlike phase verbs, modals allow for both imperfective and perfective infinitives, depending on whether you mean a process or a single completed result. Note that while words like должен (must) or нужно (need) function similarly to verbs, they are technically adjectives or predicatives.
• Phase Verbs: Verbs that indicate the start, continuation, or end of an action, such as начинать (to begin), always require an imperfective infinitive.
• Future Tense: It is used with the auxiliary verb быть to form the compound future for imperfective verbs (e.g., Я буду работать - I will work). Warning: Never use буду with a perfective infinitive; perfective verbs form the future tense via simple conjugation.
• Infinitive of Purpose: Used after verbs of motion to indicate the purpose of the movement (e.g., Я пошёл купить хлеб - I went to buy bread).
• As a Subject: It can act like a noun (e.g., Курить — вредно - To smoke is harmful).
• Strict Commands and Prohibitions: The infinitive is often used for impersonal commands or strict prohibitions (e.g., Не курить! - No smoking!, Не входить! - Do not enter!).

2. Verbal Aspect:
The vast majority of Russian verbs belong to an aspectual pair: imperfective (for ongoing or repeated actions) or perfective (for completed results). When you learn a new verb, you should learn both forms. For example, читать (imperfective) and прочитать (perfective). However, note that some verbs are biaspectual (e.g., использовать) or lack a pair entirely (e.g., знать, очнуться).

3. Spelling and Variations:
While -ть is the standard, there are two other endings:

• -ти: Used when the verb stem ends in a consonant and the ending is stressed (e.g., идти - to go). An exception is when the perfective prefix вы- is added, which always pulls the stress to itself, resulting in an unstressed -ти (e.g., выйти - to go out).
• -чь: Used when the verb root ends in a specific consonant, historically 'г' or 'к' (e.g., мочь - to be able). This helps explain why these consonants reappear in conjugation.

4. Reflexive Verbs:
If a verb is reflexive, the suffix -ся/-сь is added at the very end. Use -ся after consonants (including -ть and -чь) and -сь after vowels (including the -и in -ти). Note that the combination -ться is pronounced as a hard "ца" [tsa].
Examples: учиться (to study), беречься (to take care), пройтись (to take a stroll).