Learn Romanian: Trebuie să cumpăr o baterie.

The verb trebuie is special because it is impersonal, meaning its form doesn't change depending on who is doing the action. Instead of saying "I must", Romanian basically says "It is necessary". You don't need to put eu (I) before it.

What does the little word să do here?

In Romanian, when you connect a verb like "must" with another action, you use să between them. It translates roughly to "that". So, trebuie să cumpăr literally means "It is necessary that I buy".

If trebuie doesn't change, how do we know the sentence means "I must buy"?

The person doing the action is shown by the second verb. Here, cumpăr is the "I" form of the verb a cumpăra (to buy). If you wanted to say "you must buy", you would change the second verb to its "you" form: trebuie să cumperi.

Why do we use o instead of un for "a battery"?

The word baterie is a feminine noun. In Romanian, the word for "a" or "an" is o for feminine nouns and un for masculine or neuter nouns.