Quote Originally Posted by ironfist View Post
Why is this sentence not мой знакомый есть большой дом?
Мой знакомый есть большой дом as a sentence is using english grammar logic and Russian words. No doubt you are aware that the russian grammar system is very different to the english grammar system. This is why the senstence does not work. To understand the example senstence clearly, -у моего знакомого большой дом- большой дом is infinitive form as nothing is happening to them. However the friend or familier is becoming a possesor of something. Literally “у моего знакомого” means “by my friend” as in, “next to my friend” or in a more understandable engish “my friend has”

Now, in Russian when we use у (to mean by...) the noun and adjectives that follows it must be in a genetive case (we all own genes, right?:0) so мой in the genitive form becomes моего and знакомый becomes знакомого.

We dont have to use есть unless you want to apply some sort of significance to the statement, but that depends on the context. Imagine you were meeting old friends at a school reunion and there were rumours that one of them had a huge house, then when you see the mansion on the hill you can cast aside your doubts as you regale the occasion to your russian friends the next day “вай!! У моего знакомого есть большой дом”