Hennes väckarklocka är gammal, så hennes humör är inte alltid det bästa på morgonen.

Because Swedish makes compound nouns very freely, and they are normally written as one word.

• väckar- relates to waking someone up
• klocka means clock

So väckarklocka = alarm clock.

This is very typical in Swedish. English often uses two words where Swedish uses one:

• alarm clock → väckarklocka
• coffee cup → kaffekopp
• school book → skolbok

Because väckarklocka is an en-word in Swedish.

The adjective has to agree with the noun:

• en väckarklocka → gammal
• ett hus → gammalt
• plural → gamla

So:

• Hennes väckarklocka är gammal is correct
• gammalt would only fit an ett-word

Because Swedish, like English, normally does not use an article together with a possessive word.

So:

• hennes väckarklocka = her alarm clock
• hennes humör = her mood

Not:

• en hennes väckarklocka
• ett hennes humör

The possessive already makes the noun definite enough.

Here så means so in the sense of therefore / as a result.

It connects the two parts:

• Hennes väckarklocka är gammal
• så hennes humör är inte alltid det bästa på morgonen

So the logic is: Her alarm clock is old, so her mood is not always the best in the morning.

In this sentence, så is functioning as a coordinating connector, so the second part keeps normal main clause word order.

Because Swedish main clauses follow the V2 rule: the finite verb usually comes in the second position.

In the second clause:

• hennes humör = first element
• är = finite verb, so it comes second
• inte alltid comes after that

So:

• hennes humör är inte alltid det bästa

This is normal Swedish word order.

Also, inte alltid works as a unit meaning not always.
Putting alltid before inte would sound wrong or at least very unnatural here.

Because det bästa is a very natural Swedish way to say the best.

Here:

• bäst = superlative form meaning best
• bästa = the definite superlative form
• det bästa = the best

So:

• hennes humör är inte alltid det bästa = her mood is not always the best

This sounds idiomatic and complete.

You can sometimes see bäst used by itself in other structures, but in this sentence det bästa is the natural choice.

This is a very common question.

Swedish sin/sitt/sina is a reflexive possessive, and it refers back to the subject of the same clause.

For example:

• Hon gillar sin väckarklocka = She likes her own alarm clock

But in your sentence, the possessive appears inside the subject phrase itself:

• hennes humör

Here, humör is part of the subject, and Swedish normally does not use sin/sitt/sina that way. So hennes is correct.

Also in the first clause:

• Hennes väckarklocka är gammal

There is no reason to use sin, because the sentence is just identifying whose alarm clock it is.

So the repeated hennes is normal.

Here på morgonen means in the morning.

In Swedish, time expressions with parts of the day often use på:

• på morgonen = in the morning
• på kvällen = in the evening
• på natten = at night / during the night

In this sentence, på morgonen can sound a bit general, like in the morning / in the mornings, not necessarily one single morning only.

A very important contrast:

• på morgonen = in the morning
• i morgon = tomorrow

Those two are easy to confuse for learners.

Yes, they are historically related, but in modern usage they are not the same word in meaning.

In Swedish:

• humör usually means mood or temper

So here:

• hennes humör = her mood

If you want humor in the English sense of comedy/funniness, Swedish usually uses:

• humor

So:

• humör = mood
• humor = humor

They are easy to mix up, so this is a good word to notice.

Yes. It is the predicate after är and describes the quality of hennes humör.

The structure is:

• hennes humör = subject
• är = is
• inte alltid det bästa = is not always the best

So Swedish is basically saying: her mood is not always the best

It is a slightly idiomatic way to comment on someone’s mood, and it sounds natural in everyday Swedish.