Við erum nýbúnar að taka illgresið úr grasflötinni.

It is a very common Icelandic way to say that someone has just finished doing something.

So:

• við erum nýbúnar að taka ... = we have just finished taking ...
• very natural English equivalents are we’ve just ... or we’ve just finished ...

A literal breakdown is something like:

• við erum = we are
• nýbúnar = newly finished
• að taka = to take

But in real English, you should treat the whole structure as an idiom meaning have just done.

Because nýbúinn agrees with the subject við in gender and number.

Here, nýbúnar is:

• plural
• feminine

So this sentence is being said by a group of women, or by women speaking about themselves.

Compare:

• Við erum nýbúnar ... = we (all female)
• Við erum nýbúnir ... = we (all male or mixed group)
• Við erum nýbúin ... = neuter plural form, used in some contexts but not for ordinary we referring to people in the same way

This agreement is very important in Icelandic.