Després de dinar, necessito una pastilla perquè tinc mal de cap.

Because després normally takes de before a noun or an infinitive.

For example:

• després de classe = after class
• després de menjar = after eating
• després de dinar = after lunch / after having lunch

So de is required here.

It can be understood either way, and that is very normal in Catalan.

• As a noun, dinar = lunch
• As an infinitive verb, dinar = to have lunch

So després de dinar can mean either after lunch or after having lunch. In practice, the meaning is the same here.

Because Després de dinar is an introductory time expression at the beginning of the sentence. The comma helps separate that opening phrase from the main clause.

It is very natural and standard to write it this way: Després de dinar, necessito...

In short sentences, punctuation can sometimes vary a little, but this comma is perfectly normal.

Catalan often leaves out subject pronouns because the verb ending already shows who the subject is.

• necessito = I need
• tinc = I have

So jo is not necessary. You would add jo mainly for emphasis or contrast:

• Jo necessito una pastilla, ell no. = I need a pill, he doesn’t.

Both are first person singular present indicative forms.

• necessito comes from necessitar and is regular
• tinc comes from tenir and is irregular

So:

• necessito = I need
• tinc = I have

Here pastilla means pill or tablet, especially a medicine tablet. So una pastilla is a pill / a tablet.

Depending on context, pastilla can also refer to other small tablet-like things, but in this sentence the medical meaning is the obvious one.

Because perquè (one word, with an accent) means because.

This is easy to confuse with:

• per què = why / for what reason

So:

• Necessito una pastilla perquè tinc mal de cap. = I need a pill because I have a headache.
• Per què necessites una pastilla? = Why do you need a pill?

Catalan often uses tenir for physical states and symptoms where English may use be, have, or another structure.

For example:

• tenir gana = to be hungry
• tenir set = to be thirsty
• tenir son = to be sleepy
• tenir mal de cap = to have a headache

So tinc mal de cap is the standard, natural way to say I have a headache.

Because mal de cap is a fixed expression. In the usual form, Catalan says:

• tenir mal de cap not
• tenir un mal de cap in the neutral everyday sense

That said, you may sometimes hear un mal de cap when someone wants to emphasize a particular headache:

• Tinc un mal de cap terrible. = I have a terrible headache.

But the basic expression is tenir mal de cap.

Yes. Em fa mal el cap is also very common and natural.

Literally, it means something like my head hurts or the head hurts me. In everyday use, it is very close in meaning to tinc mal de cap.

So both are good:

• Tinc mal de cap.
• Em fa mal el cap.

In this sentence, you could also say: Després de dinar, necessito una pastilla perquè em fa mal el cap.