Breakdown of Moramo li pisati pismo?
pismo
letter
li
question particle
morati
must
pisati
to write / to say, be stated (in a text)
Questions & Answers about Moramo li pisati pismo?
Why does the verb pisati stay in its base form?
Because it follows moramo (we must / we have to). Just like in English ('we must write'), when you use a verb like morati, the action verb that comes after it stays in its dictionary form, which is called the infinitive.
How does the word li make this a question?
li is a small word used to ask yes-or-no questions. To form the question, you put the conjugated verb first (Moramo), and put li directly after it. So, Moramo li translates to 'Do we have to?'.
Where is the word for 'we' in this sentence?
The word for 'we' is mi, but you don't need it here. In Croatian, the ending of the verb moramo already shows who is doing the action. Including mi isn't necessary unless you really want to emphasize it.
Why doesn't the word pismo change its ending if it's the object of the sentence?
Because pismo (letter) is a neuter noun. It is receiving the action of writing, which means it is in the accusative case. For neuter nouns ending in -o or -e, their accusative form looks exactly the same as their basic dictionary form!