I got corrected 上手し for 上手なので, but why?

Question

I was writing a composition in Japanese, and since I wanted to give reasons I wrote the following:

僕は日本語の会話がまだできないし、友達は英語が上手し、日本語を少ししか練習しませんでした。

However, I got corrected like this:

僕は日本語の会話がまだできないし、友達は英語が上手なので、日本語を少ししか練習しませんでした。

I don't understand why I can't use し or why it's better ので in this context. Can anyone explain me the reason?

Answer 1

友達は英語が上手し、

I'm afraid 「[上手]{じょうず}し」/「[上手]{うま}し」 is incorrect.

I think you could use ～し, ～ので, or ～から here. You could say like this, using the na-adjective [上手]{じょうず}な:

[上手]{じょうず}だし、
上手なので、
上手だから、

or, using the i-adjective [上手]{うま}い:

[上手]{うま}いし、
上手いので、
上手いから、

「～し」 and 「～から」 would sound more casual than 「～ので」.

Answer 2

上手し is wrong. It must be 上手だし.

According to "A Dictionary of Basic Japanese Grammar", page 396, na-adjectives and nouns must be followed by だし (or だったし if it is the past tense).

Examples:
先生だし (noun + だし)
綺麗だし (na-adjective + だし)

Grammatically that is the only mistake.

You teacher used ので probably just to make it clear that it implies a reason or cause.
し, on the other hand, does not imply reason or cause, it is just an emphatic "and".