Well, I know they sound right to you, as they are coorect in American English (only) and you are American. Here is what a Briton would say :
"different from/to", instead of "different than"
My Oxford dictionary explains than all 3 (from, to, than) are correct, though there is a slight nuance between them. "From" is usually considered the most "correct", and "than", which is often criticised, is restricted to North America.
"We agree", instead of "we are agreed"
"Agree" cannot be used as a passive, as you are not agreed by someone.
"I've just done it", instead of "I just did it"
"You have already told me", instead of "You already told me"
More generally, the present perfect tense in British English always has a connection with the present. It is used to mean that an action continues "until now" and in a period of time that is not yet finished (today, this week, this year, this afternoon if we are still the afternoon...) or with words like "recently, so far, since, for" and so on, that have a connection with the present.
As a rule, it's "always" the present perfect with the words "just, already and yet".
There are other slight grammatical difference between AmE and BrE. For instance, Americans say "on the street", while Brits say "in the street" (what about Canadians, Aussies and others here on the forum ?)