Kiedy boli mnie łokieć, nie mogę długo pisać.

Here kiedy means when, but in this kind of general statement it often feels like whenever in English.

So Kiedy boli mnie łokieć, nie mogę długo pisać means a repeated or typical situation, not necessarily one single event.

Because Polish uses the pattern coś boli kogoś = something hurts someone.

In this sentence:

• łokieć = the elbow
• boli = hurts
• mnie = me

So the literal structure is My elbow hurts me.
That is why the verb is boli: it agrees with łokieć, which is third person singular.

Because mnie is the object form of ja.

With boleć, the person who feels the pain is usually expressed as the object:

• boli mnie głowa = my head hurts
• boli mnie ręka = my arm/hand hurts
• boli mnie łokieć = my elbow hurts

So mnie is used because it means me, not I.

Because łokieć is the subject of the verb boli, so it is in the nominative case.

The structure is:

• łokieć = subject
• boli = verb
• mnie = object

So Polish is literally saying the elbow hurts me.

Polish often leaves out possessive words like mój with body parts when the meaning is obvious.

Since the sentence says boli mnie łokieć, it is already clear that it is my elbow, not someone else’s.

Using mój would usually sound unnecessary here.

For a learner, the safest standard pattern is boli mnie łokieć.

That is the usual structure taught with boleć:
something hurts someone → person in the accusative, like mnie, cię, go.

You may hear other variants in everyday speech, but boli mnie łokieć is the form you should rely on.

Because after móc (can / to be able to), Polish uses the infinitive.

So:

• mogę pisać = I can write
• nie mogę pisać = I can’t write

You do not use a fully conjugated second verb after mogę.

Because pisać is imperfective and refers to the activity of writing in general or over a period of time.

Here the sentence says nie mogę długo pisać = I can’t write for long.
That idea of duration fits the imperfective pisać very well.

Napisać is perfective and usually focuses on completing something, like to write something up / to finish writing.

Długo is an adverb meaning for a long time or for long.

So:

• nie mogę długo pisać = I can’t write for long

It describes how long the writing can continue.

Usually pisać means to write in general.

Depending on context, it can mean handwriting or just writing more broadly. In this sentence, because the problem is an elbow, many people will naturally imagine writing by hand, but the verb itself does not have to be limited to that.

No, Polish word order is fairly flexible.

The given order is natural and neutral:

• Kiedy boli mnie łokieć, nie mogę długo pisać.

You could also move things around, but the emphasis would change. For example, putting długo earlier can stress the idea of duration more strongly.

So the sentence is not the only possible order, but it is a very normal one.

Because Kiedy boli mnie łokieć is a subordinate clause introduced by kiedy.

In Polish, subordinate clauses are separated by a comma from the main clause:

• Kiedy boli mnie łokieć, nie mogę długo pisać.

This is standard punctuation.