Learn Romanian: Trebuie să cumpăr acel cadou în noiembrie pentru decembrie. - I must buy that gift in November for December.

In Romanian, you don't use the dictionary form (infinitive) of a verb after "must" like we do in English. Instead, you use să plus a conjugated verb. You are literally saying "It is necessary that I buy." (or "It must that I buy").

If I wanted to say "He must buy," would trebuie change?

No, trebuie stays exactly the same! It is an impersonal verb, meaning its form doesn't change for I, you, he, we, or they. To change who has to do the action, you only change the verb after să. For example, "He must buy" is Trebuie să cumpere.

Why is the word for "that" acel here instead of acela?

When the word for "that" comes right before the noun, you drop the final "a", making it acel for masculine and neuter nouns like cadou. The longer form acela is used when the word stands alone as a pronoun ("I want that one") or comes after the noun (cadoul acela).

Why are noiembrie and decembrie written with lowercase letters?

Unlike in English, months of the year and days of the week are not capitalized in Romanian unless they appear at the very beginning of a sentence.