Ne pišite gluposti u motivacijskom pismu.

Because pišite is the imperative form of pisati: write!

Compare:

• pišete = you write / you are writing (present tense)
• pišite = write! (imperative)

So Ne pišite... means Don’t write..., not You do not write...

Pišite is used for:

• you plural: when talking to more than one person
• formal/polite you singular: when addressing one person respectfully

Piši is the informal singular form, used with one person you know well.

So:

• Ne piši gluposti. = Don’t write nonsense. (to one friend / informal)
• Ne pišite gluposti. = Don’t write nonsense. (to several people, or to one person formally)

It can mean either:

1. Don’t write (to several people)
2. Don’t write (to one person, politely/formally)

Croatian, like many European languages, uses the same verb form for plural you and formal you.

Gluposti literally means stupid things, but in sentences like this it often means:

• nonsense
• stupid stuff
• ridiculous things

So Ne pišite gluposti is a very natural way to say Don’t write nonsense or Don’t write stupid things.

In Croatian, the plural is often used where English would simply say nonsense or stupid stuff.

The singular glupost means:

• a stupid thing
• a foolish idea
• nonsense (in some contexts)

But gluposti is very common when talking about foolish or ridiculous content in general.

So pisati gluposti is a common expression meaning to write nonsense / to write rubbish.

It is in the accusative plural, because it is the direct object of pišite.

You are writing what?
→ gluposti

The noun is glupost (feminine), and its plural form gluposti happens to look the same in nominative plural and accusative plural.

Here u means in, so it takes the locative case.

That is why you get:

• motivacijsko pismo = a motivation letter / cover letter (nominative)
• u motivacijskom pismu = in the motivation letter (locative)

A useful rule:

• u + locative = in / at (location)
• u + accusative = into (movement toward the inside of something)

Here there is no movement, just location/context: in the letter.

Because adjectives in Croatian must agree with the noun they describe.

The noun is:

• pismo = letter, a neuter singular noun

In the phrase u motivacijskom pismu, the whole noun phrase is in the locative singular, so both words change:

• motivacijsko pismo (nominative singular)
• u motivacijskom pismu (locative singular)

So motivacijskom matches pismu in:

• gender: neuter
• number: singular
• case: locative

Yes. Motivacijskom and motivacijskome are both possible.

• u motivacijskom pismu = very common
• u motivacijskome pismu = also correct, a bit fuller or sometimes slightly more formal/literary in feel

Learners will hear and read both types of endings in Croatian adjectives.

Yes. That is also correct and very natural.

Both mean essentially the same thing:

• Ne pišite gluposti...
• Nemojte pisati gluposti...

The second version uses nemojte + infinitive, which is a very common way to make a negative command.

Very roughly:

• Ne pišite... = more direct Don’t write...
• Nemojte pisati... = also Don’t write..., often felt as a standard and common prohibitive form

This is about aspect.

• pisati = to write (imperfective: ongoing, general activity)
• napisati = to write / to finish writing (perfective: completed result)

Here the point is don’t engage in writing nonsense, so the imperfective verb pisati is the natural choice.

Compare the ideas:

• Ne pišite gluposti. = Don’t write nonsense.
• Napišite pismo. = Write the letter. (finish it / produce it)

So pisati fits better in this warning or instruction.