Questions & Answers about Мы должны пить воду.
Why is должны used here, and what does it mean?
должны is the short-form plural of the adjective должен, functioning as a modal verb meaning “must” or “have to.” Short-form adjectives in Russian agree in number and gender with the subject and can express necessity. Since мы (“we”) is plural, you use должны rather than должен (masculine singular) or должна (feminine singular).
Why is пить in the infinitive form instead of a conjugated verb?
After modal expressions like должны, Russian uses the infinitive to indicate the required action. Here пить (“to drink”) remains in the infinitive, just as in English you say “must drink” rather than “must we drink” with a conjugated form.
Why is воду in the accusative case, and not вода?
The verb пить (“to drink”) takes a direct object in the accusative case. Feminine nouns ending in -а change that ending to -у in the accusative, so вода becomes воду.
Could we drop мы and just say должны пить воду?