Hún heldur að það sé stífla í sturtunni líka, því vatnið fer ekki niður.

Sé is the present subjunctive form of vera (to be).

In a sentence like Hún heldur að ..., Icelandic often uses the subjunctive in the að-clause because the speaker is reporting a belief, assumption, or suspicion rather than a confirmed fact.

So:

• að það sé stífla = that there may be / is apparently a clog
• að það er stífla would sound more definite and factual

In everyday speech, you may sometimes hear er, but sé is very natural and standard here.

Here það is a kind of dummy subject, similar to English there in there is a clog.

So Icelandic says:

• það sé stífla
  literally something like it be a clog

But the real meaning is:

• there is a clog

This það does not refer to a specific thing. It is just there because Icelandic often needs a subject-like word in this structure.

Because í takes the dative when it expresses location.

Here, í sturtunni means in the shower, so it is about where the clog is located, not motion into the shower.

Breakdown:

• sturta = shower
• sturtunni = the shower in the dative singular

So:

• í sturtunni = in the shower

If you are learning cases, this is a very common pattern:

• í + dative for location
• í + accusative for motion into something

Stífla means a blockage, clog, or obstruction.

In this sentence it refers to a drain blockage, not necessarily something visible in the shower itself. So stífla í sturtunni naturally means there is probably a clog in the shower drain / plumbing.

It is a feminine noun:

• stífla = a clog
• stíflan = the clog

Yes. Halda has several meanings, and one of them is to think / believe / suppose when followed by að.

So:

• halda á einhverju = hold something
• halda að ... = think / believe that ...

In this sentence:

• Hún heldur að ... = She thinks that ...

This is a very common use of halda.

Líka means also / too / as well.

Here it most naturally means that the shower is another place with a problem:

• she thinks there is a clog in the shower too

So maybe there was already some other blockage mentioned earlier, and now the speaker adds the shower to the list.

In Icelandic, líka can move around a bit, and its position can slightly change emphasis, but here it attaches naturally to the idea of there being a clog in the shower as well.

Here því is being used as a conjunction meaning because / since / for.

So:

• því vatnið fer ekki niður = because the water is not going down

This use is common in written and spoken Icelandic, though learners will also often meet:

• af því að = because

Both can introduce a reason. In this sentence, því is short and natural.

Literally, it means:

• the water goes not down

Natural English would be:

• the water doesn’t go down
• the water isn’t draining

The expression fara niður often means go down, but with water, drains, toilets, and sinks it commonly means drain away properly.

So this is a very practical everyday phrase.

Because this is a subordinate clause introduced by að.

In main clauses, Icelandic often has verb-second word order, but subordinate clauses are different. After að, the subject or subject-like element usually comes before the verb:

• að það sé stífla

That is the normal order here.

So compare:

• Main clause style: verb-second is common
• Subordinate clause after að: subject before the finite verb is normal

Vatnið means the water.

Icelandic often uses the definite form in places where English might or might not use the, especially when talking about a specific substance already present in the situation. Here it means the water in the shower.

Breakdown:

• vatn = water
• vatnið = the water

So the sentence is talking about the specific water that should be draining away.

Yes, it would still be understood.

If someone said:

• Hún heldur að það er stífla ...

people would understand it. But sé is the more standard and grammatically expected choice when reporting what someone thinks or suspects.

So for a learner, the safest takeaway is:

• after heldur að ..., expect the subjunctive quite often
• here that gives you sé

Grammatically it says in the shower, but in real-life meaning it usually refers to the shower drain or plumbing.

That is normal. Languages often name the larger location instead of the exact pipe part involved.

So:

• stífla í sturtunni does not mean the shower cabin itself is blocked
• it means there is a blockage affecting the shower drain

That is a very natural way to say it in Icelandic.