Þegar lokið er tekið af, kemur meiri gufa út.

Þegar means when here and introduces a time clause.

So the sentence is divided into two parts:

• Þegar lokið er tekið af = when the lid is taken off
• kemur meiri gufa út = more steam comes out

It works much like English when at the beginning of a clause.

Lokið is the definite singular form of lok, which means lid or cover.

So:

• lok = a lid
• lokið = the lid

The ending -ið is the neuter singular definite article attached to the noun.

This sentence uses the passive voice:

• lokið er tekið af = the lid is taken off

In Icelandic, this is a very normal way to say something when the doer is not important or not mentioned.

Compare:

• Maðurinn tekur lokið af. = The man takes the lid off.
  (active)
• Lokið er tekið af. = The lid is taken off.
  (passive)

The sentence chooses the passive because the focus is on what happens to the lid, not on who removes it.

Tekið is the past participle of taka (to take), used here in a passive construction with er.

So:

• er tekið = is taken

It appears as tekið because it agrees with lokið, which is:

• singular
• neuter
• definite

This kind of agreement is very common in Icelandic passives.

Here af means off.

The combination taka af means take off or remove.

So:

• taka lokið af = take the lid off
• lokið er tekið af = the lid is taken off

This is one of those verb + particle combinations that often has to be learned as a unit.

This is because Icelandic has a strong verb-second pattern in main clauses.

The whole first clause:

• Þegar lokið er tekið af

counts as the first element of the sentence. After that, the finite verb comes next:

• kemur

So the main clause is ordered like this:

1. Þegar lokið er tekið af
2. kemur
3. meiri gufa
4. út

That is why you get:

• Þegar lokið er tekið af, kemur meiri gufa út.

If the sentence began directly with meiri gufa, then you could say:

• Meiri gufa kemur út.

Because gufa is a feminine noun.

Meiri is the comparative form meaning more, and it changes form to match the noun. Since gufa is feminine singular, the correct form here is:

• meiri gufa = more steam

If the noun were neuter, you would often see meira instead.

Út means out.

It completes the idea of movement outward:

• gufa kemur út = steam comes out

Without út, the sentence would still be understandable, but út makes the meaning more precise and natural: the steam is coming out from inside something.

It is a present passive construction.

Even though tekið is a past participle, the whole phrase er tekið af means:

• is taken off

not was taken off

The tense comes from er:

• er tekið af = is taken off
• var tekið af = was taken off

So in your sentence, the meaning is present/general:

• When the lid is taken off, more steam comes out.

Here gufa is being used like a mass noun, similar to English steam.

So meiri gufa means:

• more steam

not more steams

This is why the sentence does not need a plural noun here.

Yes, that literal breakdown is helpful for learning the structure:

• Þegar = when
• lokið = the lid
• er tekið af = is taken off
• kemur = comes
• meiri gufa = more steam
• út = out

But in natural English, you would say:

• When the lid is taken off, more steam comes out.

So the Icelandic word order is not always the same as English, even when the meaning matches closely.