Ég tæmi glasið áður en ég fer heim.

Tæmi comes from the verb að tæma, which means to empty, to drain, or to finish off.

In this sentence, Ég tæmi glasið means something like:

• I empty the glass
• I finish the drink in the glass

So it can be literal, or it can imply finishing what is in the glass.

It is 1st person singular present tense:

• ég tæmi = I empty / I finish

The dictionary form is að tæma = to empty.

So the pattern is:

• að tæma = infinitive
• ég tæmi = I empty
• þú tæmir = you empty
• hann/hún/það tæmir = he/she/it empties

Because glasið means the glass, while glas means a glass or just glass in an indefinite sense.

Icelandic usually adds the definite article to the end of the noun:

• glas = a glass
• glasið = the glass

So Ég tæmi glasið specifically means I empty the glass.

It is the direct object of tæmi, so it is in the accusative case.

The verb að tæma normally takes an accusative object:

• tæma glas = to empty a glass

In this particular noun, the nominative and accusative singular forms happen to look the same:

• glasið can be nominative singular definite
• glasið can also be accusative singular definite

So the case is determined by the sentence role, not by a different spelling here.

Áður en means before.

It introduces a clause:

• áður en ég fer heim = before I go home

Even though en often means than or but in other contexts, here áður en works together as a fixed expression meaning before.

Because fer is the conjugated verb form that matches ég.

• að fara = to go
• ég fer = I go

After ég, you need the finite verb form, not the infinitive:

• correct: ég fer
• not correct here: ég fara

This is just one of the forms you need to memorize for að fara, since it is an irregular verb.

Because the sentence has two clauses, and each clause needs its own subject.

• Ég tæmi glasið = I empty the glass
• áður en ég fer heim = before I go home

English does the same:

• I empty the glass before I go home

You cannot normally leave out the second ég just because the subject is the same.

Because heim is a very common adverb meaning home in the sense of homewards / to home.

So:

• ég fer heim = I go home

Compare:

• heim = home, towards home
• heima = at home
• að heiman = from home

So in this sentence:

• ég fer heim = I go home not
• I go at home

Grammatically, it is present tense, but it can easily refer to the near future or a planned action.

So Ég tæmi glasið áður en ég fer heim can mean:

• I am finishing the glass before I go home
• I’ll finish the glass before I go home

This is very normal in Icelandic, just as English says:

• before I go home not usually
• before I will go home

It is a very normal word order.

Main clause:

• Ég tæmi glasið = subject + verb + object

Subordinate clause after áður en:

• ég fer heim = subject + verb + adverb

So the full sentence is straightforward:

• Ég tæmi glasið áður en ég fer heim.

You could also move the time clause to the front:

• Áður en ég fer heim, tæmi ég glasið.

In that version, the main clause changes word order because Icelandic likes the finite verb early in the clause:

• tæmi ég rather than ég tæmi after the fronted clause.