The girl finds herself alone and does not understand what has happened.

The girl, who had been looking on in terror, understood at once.

They spoke to one another in a language that the girls could not understand.

We girls understand these things better than you men do.

I doubt if the girl would even understand that word.

Oh yes, this little girl understood a thing or two.

After a few minutes the girls understood that I was calling.

The girls could finally understand something which was going on.

Because they were concepts rather than words, the girl could understand.

Perhaps the girl would understand, finally, that all of this was inevitable.