"Yesterday he drank eleven cups of coffee."

Translation: Včera vypil jedenáct šálků kávy.

[eimre]

why is "vcera on vypil..." incorrect?

[VladaFu]

Sounds strange, the pronoun is superfluous and sounds strange at the second position, which is unstressed. It might be marginally acceptable, but even if accepted, it will be strange. Normally use just "Včera vypil...". You can also use "On včera vypil..." which stresses on somewhat.

If you want to strongly stress it was HIM how did it, you rather put on at the end of the sentence or at the end. "Včera vypil ... on.".

[BoneheadBass]

I'm getting the impression that pronouns, as subjects, are very often (though not always) left out, since in many cases they're not needed, because the verb does the work. So I've been trying to get into the habit of leaving them out as well.