Jag nickade bara, eftersom jag var för trött för att svara och började gäspa igen.

Nickade is the past tense, so it matches the finished action in the sentence.

The verb forms are:

• nicka = to nod
• nickar = nods / is nodding
• nickade = nodded
• nickat = nodded, as a supine form used with har: har nickat

So Jag nickade bara means I just nodded.

In a simple main clause, Swedish often places bara after the finite verb:

• Jag nickade bara

This is a very natural way to say I only / just nodded.

You may also hear Jag bara nickade, especially in speech, but Jag nickade bara is a neutral, standard placement.

Eftersom means because or since and introduces a subordinate clause.

That is why you get:

• eftersom jag var ...

and not:

• eftersom var jag ...

In Swedish, subordinate clauses do not use the main-clause inversion pattern. So after eftersom, the subject normally comes before the finite verb.

A useful comparison:

• Main clause: Jag var trött
• Subordinate clause: ... eftersom jag var trött

They are doing two different jobs.

• The first för means too
• för att introduces the infinitive phrase to answer

So the pattern is:

• för + adjective + för att + infinitive

Examples:

• för ung för att köra = too young to drive
• för dyr för att köpa = too expensive to buy
• för trött för att svara = too tired to answer

So even though it looks repetitive, it is a normal and common Swedish structure.

Because these are two different constructions.

In för att svara, the att is required as part of the expression för att.

But after börja, Swedish usually uses the infinitive without att:

• började gäspa
• började läsa
• började springa

You may sometimes see började att gäspa, but började gäspa is the more common modern choice.

Because the subject is still the same.

In Swedish, just like in English, if two verbs are coordinated with och and have the same subject, you usually do not repeat it:

• ... eftersom jag var för trött för att svara och började gäspa igen

The subject jag applies to both var and började.

If the subject changed, then Swedish would need to state it clearly.

Igen usually comes after the verb or verb phrase it belongs to.

So:

• gäspa igen = yawn again
• började gäspa igen = started yawning again

That is the normal word order here.

Putting igen somewhere else would usually sound unnatural or would change the emphasis.

Not always.

In modern Swedish, commas are often used less than in English. A comma before eftersom can be added for clarity, especially if the second part is long, but many writers would also accept the sentence without it:

• Jag nickade bara eftersom jag var för trött för att svara och började gäspa igen.

So the comma is possible and natural, but not strictly required in every case.

Because here svara means to answer / respond in a general sense, without saying what was being answered.

Swedish often allows that:

• Jag orkade inte svara = I didn’t have the energy to answer / reply

If you want to mention the thing being answered, Swedish often uses på:

• svara på en fråga = answer a question
• svara på ett mejl = reply to an email

So in this sentence, svara alone is perfectly natural.

Not in the same way.

Here eftersom means because, introducing the reason:

• ... eftersom jag var för trött ... = because I was too tired ...

But för att usually means to / in order to, or sometimes because in more informal spoken Swedish. In this sentence, using eftersom is the clearest and most standard written choice for because.

Also, the sentence already contains för att svara, where för att means to answer, so replacing eftersom would make the structure less clear.