Ég bremsa fyrr þegar ég sé börn við gangbrautina.

Bremsa means to brake or to put on the brakes.

In this sentence, ég bremsa means I brake. It looks the same as the infinitive að bremsa because many Icelandic verbs have the same form in the infinitive and the 1st person singular present.

So:

• að bremsa = to brake
• ég bremsa = I brake

The subject ég shows that this is a finite verb form, not the infinitive.

Fyrr means earlier or sooner.

Here it works as an adverb modifying bremsa, so Ég bremsa fyrr means I brake earlier.

It is the comparative form, so it expresses doing something earlier than usual or earlier than in another situation.

Because sjá is the infinitive, meaning to see, but after ég you need the conjugated present-tense form.

The verb sjá is irregular:

• að sjá = to see
• ég sé = I see

So:

• þegar ég sé börn = when I see children

Using sjá there would be like saying when I to see children, which is not grammatical.

Not always. Sé can also appear as a form of vera in other contexts, but in this sentence it is clearly from sjá because it is followed by a direct object, börn.

So here:

• ég sé börn = I see children

That makes sé the present form of sjá.

Börn is the plural of barn (child).

So:

• barn = child
• börn = children

This is an irregular plural, so it does not form the plural in a simple predictable way.

Because börn is the indefinite plural: children, not the children.

So:

• börn = children
• börnin = the children

In this sentence, the speaker is talking about children in general who are at the crossing, not necessarily a specifically identified group already mentioned.

It actually is functioning as the direct object of sé, so it is in the accusative. But for the noun barn, the nominative plural and accusative plural are both börn.

So even though it is an object, the form stays:

• nominative plural: börn
• accusative plural: börn

That is why English speakers often do not notice any case change here.

Here við gangbrautina means at the pedestrian crossing / by the crosswalk.

The preposition við has several meanings depending on context, such as by, at, with, or against. In this sentence, it means something like at or by.

And:

• gangbraut = pedestrian crossing / crosswalk
• gangbrautina = the pedestrian crossing / the crosswalk

So börn við gangbrautina means children at the crosswalk.

That ending includes the definite article and the case ending.

In Icelandic, the is usually attached to the end of the noun rather than written as a separate word.

Base noun:

• gangbraut = pedestrian crossing / crosswalk

Definite form here:

• gangbrautina = the pedestrian crossing / the crosswalk

The form is accusative singular definite, because við takes the accusative.

So the ending is not random: it shows both case and definiteness.

Because the clause after þegar is a full subordinate clause with its own subject and verb:

• þegar ég sé börn við gangbrautina = when I see children at the crossing

English does the same thing:

• I brake earlier when I see children...

You still need I in the when-clause, and Icelandic also needs ég.

After a subordinating conjunction like þegar (when), Icelandic normally has regular subordinate-clause order, with the subject before the verb:

• þegar ég sé börn...

That is why you get ég sé, not an inverted order.

A useful comparison:

• Ég bremsa fyrr þegar ég sé börn við gangbrautina.
• Þegar ég sé börn við gangbrautina, bremsa ég fyrr.

In the second version, the when-clause comes first, so the main clause shows the usual Icelandic inversion: bremsa ég fyrr.

Normally it sounds like a general habit or usual reaction:

• I brake earlier when I see children at the crosswalk.

The present tense in Icelandic, like in English, is often used for things that are generally true or habitual.

So the sentence suggests something like: Whenever this situation happens, I tend to brake earlier.

Yes. Icelandic often uses the definite form where English may use either the or sometimes a more natural indefinite expression depending on context.

So við gangbrautina literally contains the, but in natural English the best translation might be:

• at the crosswalk
• at a crosswalk
• at the pedestrian crossing

The exact English choice depends on context and style, even though the Icelandic form itself is definite.