Ég tek áttavitann upp úr vasanum þegar við erum ekki alveg viss um leiðina.

Tek is the 1st person singular present tense of taka (to take).

• ég tek = I take
• infinitive: taka
• past tense: tók = took

So Ég tek ... means I take ... or I get ..., depending on context.

Because it is a definite direct object in the accusative singular.

The noun is áttaviti (compass), and in this sentence it is the compass, not just a compass. Icelandic usually attaches the to the end of the noun instead of using a separate word.

For this noun:

• nominative singular definite: áttavitinn
• accusative singular definite: áttavitann

It is accusative because it is the thing being taken.

In Icelandic, the definite article is usually attached to the noun.

So instead of a separate word like English the, you get endings such as:

• áttavitann = the compass
• vasanum = the pocket
• leiðina = the route / the way

Both the noun and the article reflect case, number, and gender, which is why the endings change.

Upp úr is the natural way to express out of in a sentence like this.

• úr means out of / from
• upp adds the idea of something being brought up/out

So taka eitthvað upp úr einhverju means to take something out of something.

In English, we just say take ... out of ..., but Icelandic often uses this two-part combination.

Because Icelandic verb particles can come after the object.

So:

• Ég tek áttavitann upp úr vasanum is natural.

The particle upp belongs with the verb idea (take out), but it does not have to stand right next to tek. This is similar to English phrasal verbs in sentences like I took the compass out of my pocket.

Because úr takes the dative case.

The noun is vasi (pocket), and here it appears as vasanum, which is the dative singular definite form.

So:

• úr vasa = out of a pocket
• úr vasanum = out of the pocket

A very common thing to learn in Icelandic is which prepositions take which case, and úr takes dative.

Þegar introduces a time clause and means when.

So the sentence is structured like this:

• main clause: Ég tek áttavitann upp úr vasanum
• subordinate clause: þegar við erum ekki alveg viss um leiðina

In other words: I take the compass out of my pocket when ...

Because after a subordinating conjunction like þegar, Icelandic normally uses subordinate clause word order, where the subject comes before the finite verb.

So:

• þegar við erum ... = correct
• þegar erum við ... = not the normal structure here

This is different from main-clause verb-second patterns, which are very important in Icelandic.

In Icelandic, ekki usually comes after the finite verb.

So:

• við erum ekki = we are not

That is the normal position in a sentence like this. English learners often expect the negative word to behave differently, but in Icelandic vera + ekki is very common.

Alveg means something like completely, entirely, or quite, depending on context.

In the phrase ekki alveg viss, it gives the meaning not quite sure.

So:

• ekki viss = not sure
• ekki alveg viss = not completely / not quite sure

It makes the statement a little softer and more natural.

Because vera viss um eitthvað is the normal Icelandic expression for to be sure about something.

Here:

• viss = sure/certain
• um = about

So við erum ekki alveg viss um leiðina means we are not quite sure about the route / about the way.

This is a fixed and very common pattern, so it is best learned as a whole expression.

Because the preposition um takes the accusative case here.

The noun is leið (way, route, direction), and leiðina is the accusative singular definite form.

So:

• um leið = about a route/way
• um leiðina = about the route/way

Again, this is part of learning which case each preposition requires.

Because the speaker is talking about a specific route or way that both people already have in mind.

Icelandic often uses the definite form when the thing is understood from the situation. So leiðina here is very natural: it means the route / the way we are trying to follow.

English might sometimes be a bit looser here, but Icelandic often makes that specificity clearer.

This is a good question, because learners often expect full adjective agreement.

In everyday Icelandic, viss in the expression vera viss um is very often used as a kind of fixed predicate form, especially in speech, so við erum ekki alveg viss sounds natural. You may also come across agreeing plural forms such as vissir or vissar, especially in more careful or formal usage, depending on who við refers to.

So the form in your sentence is normal and idiomatic.