English happily ends a sentence with a preposition. In Spanish that is simply incorrect, so the sentence gets rearranged.
It sounds more formal in English, but in Spanish it is the only correct option, not a register choice.
No sé con cual quiere hacerlo · No sé con quien tengo que hacerlo · No sé para quien tengo que hacerlo · No sé por quien tengo que hacerlo
For the girl I work with, Spanish says the girl with that I work. You have several options for that middle word, and they are all correct.
La chica con que trabajo · con cual · con la cual · con la que · con quien
Es la chica con que trabajaba · La chica con que iba a la fiesta
You need no word for one: ¿Cuál quieres? is which one do you want?
Quién goes with the he/she/it form of the verb. Spanish also has a plural who, for when you know your who refers to more than one person, and the verb matches.
¿Quién va? · ¿Quiénes van? · ¿Quién está aquí? · No sé cual quiere · No sé cual quiere ver
Since estar carries location, ¿Quién está? alone is enough when the here is understood.
Meeting Estados Unidos, you can work backwards from unidos to find the verb. The instruction is precise: not guessing, but seeing what has been added and consciously taking it off.
unidos → strip -ido → unir
That leaves a choice between -er and -ir. Experiment and see which feels more natural. A version you have heard before will feel right even if you never registered it.
Vamos a Estados Unidos, with no word for the.
The line past of ir is irregular, and there are two routes to it.
In the present it is spelled with a v, in the past with a b, but they sound so alike that either hook works.