Modal verbs and their complement: infinitive or da plus present

Some verbs do not say what happens — they say something about what happens: that you can do it, that you must do it, that you want to do it. In Serbian these verbs behave just like any other verb: they take the ordinary present endings you already know. The new thing is what comes after them.

Two ways to finish the sentence

After a modal verb the main verb can appear in two shapes, and both are completely normal Serbian:

• the infinitive — the dictionary form: Moram raditi. (I have to work.)
• da + the present tense — the main verb conjugated for the same person: Moram da radim. (I have to work.)

They mean the same thing. In Serbia the second one, with da, is what you hear most in speech, so it is the safer default; the infinitive sounds a little more written or formal. Notice that with da the second verb also carries the person: moram da radim, moraš da radiš, moramo da radimo.

Every modal takes both shapes: Želim da idem. / Želim ići. (I want to go.) — Ne smeš da pušiš. / Ne smeš pušiti. (You mustn't smoke.) — Ona zna da pliva. (She knows how to swim.)

The modal is the verb that carries the person. Get that one right and the rest follows: moram da radim, možeš da radiš, oni mogu da rade.

The verbs themselves

Each modal belongs to a class you have already met, so there is nothing new to learn about the endings.

Two more belong here. znati (to know) means to know how to when a verb follows it, and trebati means to need to, to ought to. In everyday Serbian both of these are almost always finished with da, not with the infinitive.

• Znam da kuvam. — I know how to cook. (Znam kuvati exists, but sounds Croatian.)
• Treba da učim. — I ought to study. Here treba stays in the third person for everybody: treba da učiš, treba da učimo.
• Treba mi odmor. — I need a rest. With a noun too, trebati stays in the third person: the thing needed is the subject, and the person who needs it goes into the dative — treba mi pomoć, trebaju mi ključevi.

Saying no

A modal is negated the way any verb is: ne goes in front of it. But the English translations pull apart, and this catches everybody:

English I must not is ne smem, never ne moram. If you say Ne moram da idem, you are saying you are free to stay — not that going is forbidden.

Asking permission

To ask whether you may do something, use moći or smeti in a question, exactly as you learned with da li: Mogu li da uđem? or Da li mogu da uđem? — May I come in? A short Možeš. or Ne možeš. is a complete answer.