Na seznam jsem zapomněl napsat mrkev, proto jsem se musel vrátit do obchodu.

Because Czech usually drops subject pronouns when they are clear from the verb form.

• jsem zapomněl = I forgot
• jsem se musel vrátit = I had to return

So já is not necessary. You could add it for emphasis, but normally you would leave it out.

In Czech, short unstressed words called clitics usually go in second position in the clause. The auxiliary jsem is one of them.

So in:

Na seznam jsem zapomněl napsat mrkev

the first unit is Na seznam, and then jsem comes right after it.

This is very normal Czech word order. English speakers often expect the verb to stay together, but Czech often splits it up.

Czech word order is flexible, and the beginning of the sentence often shows the topic or the part the speaker wants to frame first.

Starting with Na seznam puts focus on the list itself, roughly like:

• As for the list, I forgot to write carrots on it

A more neutral order would also be possible, for example:

Zapomněl jsem napsat mrkev na seznam.

Both are grammatical, but the original sounds natural and slightly more contextual.

Because Czech uses na with the idea of writing something onto a surface or place such as a list, board, paper, sign, and so on.

So:

• napsat něco na seznam = to write something on a list

Here seznam is in the accusative after na, because there is movement toward the list as the target of writing.

This is a very common Czech structure:

• zapomenout + infinitive = to forget to do something

So:

• zapomněl napsat mrkev = forgot to write carrots/carrot

This is the normal way to say forgot to write in Czech.

Because napsat is the perfective form, meaning a complete, finished act of writing something down.

• psát = to be writing / to write in a general or ongoing sense
• napsat = to write down, to complete the writing

Here the speaker forgot to complete one specific action: putting mrkev on the list. So napsat is the natural choice.

It is the direct object of napsat, so it is in the accusative.

However, for mrkev, the accusative singular looks the same as the nominative singular:

• nominative: mrkev
• accusative: mrkev

So even though the case changes functionally, the form does not visibly change here.

Also, Czech has no articles, so there is no word for a or the before mrkev.

That is the masculine singular past form, so the sentence is spoken by a male speaker.

• male: zapomněl, musel
• female: zapomněla, musela

So if a woman said the same sentence, it would be:

Na seznam jsem zapomněla napsat mrkev, proto jsem se musela vrátit do obchodu.

Because vrátit se means to return / to go back.

Without se, vrátit usually means to return something:

• vrátit se do obchodu = to go back to the shop
• vrátit zboží = to return goods/an item

So se is necessary here because the subject is returning himself/herself.

In the actual sentence it is:

proto jsem se musel vrátit

Here both jsem and se are clitics, and they usually come near the start of the clause in a fixed order. So Czech naturally places them before the main participle and infinitive.

That is why you get:

• proto jsem se musel vrátit

rather than trying to keep everything together as English would.

Because do means into / to and it requires the genitive case.

• dictionary form: obchod
• genitive after do: obchodu

So:

• do obchodu = to the shop / into the shop

This is used with motion toward the inside of a place.

Proto here means therefore, that is why, or so.

It connects the two clauses:

• I forgot to write carrots on the list,
• therefore I had to go back to the shop.

The comma is used because these are two separate clauses joined in a logical relationship. That punctuation is standard in Czech.

Czech often uses the singular for food items in a general, item-like sense, especially on lists.

So mrkev can work naturally as a grocery-list item, even if you would buy more than one carrot. English often prefers the plural here, but Czech does not have to match that exactly.