Pošto sam zaboravila pernicu kod kuće, danas sve pišem jednom starom kemijskom.

Here pošto means since or because.

So the first part, Pošto sam zaboravila pernicu kod kuće, gives the reason for what follows.

Be aware that pošto can have other meanings in other contexts, such as after or for what price/how much, but in this sentence it clearly means because/since.

Sam zaboravila is the Croatian perfect tense, used here for I forgot.

It is made from:

• sam = I am (auxiliary form of biti, used to build the past)
• zaboravila = past participle of zaboraviti

The form zaboravila also tells you the speaker is female.
A male speaker would say zaboravio.

So:

• sam zaboravila = I forgot said by a woman
• sam zaboravio = I forgot said by a man

Croatian often leaves out subject pronouns when they are already clear from the verb form.

Here sam already shows first person singular, so ja is unnecessary.

You could say Pošto sam ja zaboravila..., but that would sound more emphatic, as if stressing I in particular.

Because pernicu is the accusative singular form of pernica.

Pernica is the direct object of zaboravila:

• zaboraviti što? = to forget what?
• pernicu

So:

• nominative: pernica
• accusative: pernicu

This is very common in Croatian: feminine nouns ending in -a usually change to -u in the accusative singular.

Kod kuće means at home.

It is a fixed, very common expression:

• kod
  • genitive
• kuće is the genitive singular of kuća

A useful contrast:

• kod kuće = at home
• u kući = in the house

So kod kuće is more idiomatic and refers to being at one’s home in general, not just physically inside a building.

Sve means everything or all of it.

So danas sve pišem means today I’m writing everything.

Here sve is the object of pišem:

• pišem što? = what am I writing?
• sve

The word order is flexible in Croatian. This sentence could also be rearranged somewhat, but danas sve pišem sounds natural and keeps today and everything nicely in focus.

Grammatically, jednom is the instrumental singular form of jedan.

Yes, it literally means one, but in sentences like this it often works a bit like English a/an, giving the sense of with an old pen rather than strongly emphasizing the number one.

It agrees with kemijskom, which is feminine here:

• jedna stara kemijska = an old pen
• jednom starom kemijskom = with an old pen

So it can mean:

• literally: with one old pen
• more naturally in English: with an old pen

Croatian often uses the instrumental case by itself to show the tool or means used to do something.

So:

• pišem olovkom = I write with a pencil
• pišem kemijskom = I write with a pen

That is why there is no separate word for with here. The meaning with is built into the instrumental case.

So jednom starom kemijskom is in the instrumental because it tells you what she is writing with.

Kemijska in everyday Croatian means ballpoint pen. It is a common short form of kemijska olovka.

In this sentence, kemijskom is the instrumental singular form:

• nominative: kemijska
• instrumental: kemijskom

So jednom starom kemijskom means with an old pen.

You could also say the fuller version jednom starom kemijskom olovkom, but in normal speech olovkom is often left out because kemijska already clearly means pen.

Yes. The comma is standard here because the sentence begins with a subordinate clause:

• Pošto sam zaboravila pernicu kod kuće, = dependent clause
• danas sve pišem jednom starom kemijskom. = main clause

When the reason clause comes first, Croatian normally separates it from the main clause with a comma.