French Grammar Question?

[Icobes]

For the following sentences, I needed to replace the bolded words with a personal pronoun:

1) Comment sais-tu si elle (bolded) est sortie pendant la récréation.

I responded with the following sentnece:

Je l'ai vue sortir pendant la récréation.

I knew elle should be replaced by la, which is what I did.

For this sentence however, it is a bit different.

The question was: COmment sais-tu que nous (bolded) avons écrit une lettre au ministre.

I responded with:

Je les ai vus écrire une lettre au ministre.

However that was wrong. My teacher said that instead of les, you must use vous, so that it would be Je vous ai vus écrire...

Why is that? Also I notice it is the same thing with je, since it is replaced with te. Would this also therefore apply to me replacing tu with me and vous with nous? I am really confused about this and I hope you can explain why I must do it and in which cases.

- Thanks for the help
Also in this sentence:

Je vois mes amis (bolded) participer au sondage...

I replaced it with

Je les vois participer...

Why would it not be nous or vous since nos means ours? Why would les be right here but not in my example above?

- THanks