"I know these cities."

Translation: Я знаю эти города.

[JewishPolyglot]

Interesting... Why is the plural of город города instead of городи or something like that?

[AntoineFab]

I got it wrong as well :), it's just an irregular form, the most common ones are given on this page : http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit2/GramUnit2_2.htm

[JewishPolyglot]

Спасибо!

[lgularte]

Can we leave out "Я"?

[guido506552]

This sentence is not at its place, it seems (город never presented in plural, nor plural accusative)

[jQuasebarth]

Does "знаю" require the accusative case?

[elsantodel90]

Yes, but for plural inanimate nouns it is just the same as the nominative. Note that "города" here is not the singular accusative of "город", but the plural accusative (which is the same as the nominative, and it is an irregular form).