They're not really interchangeable in this case. "I see the wood." would be referring to a pile of logs or scraps of wood. "I see the woods." is how you would refer to a group of trees.
No — “I see the wood” can also refer to a group of trees, for many English-speakers.
Jag ser inte skogen för alla träd. Various expressions with inte se skogen för alla träd are pretty widely used in Swedish too.
How do I predict the "sj" and "sk" sound that is tipically swedish? (the one in skjorta)
As a rule of thumb, the 'Swedish' pronunciation (as in skjorta) applies for
sch, sj, skj, stj - regardless what vowel follows
sk - if followed by e, i, y, ö or ä
-sion, -tion endings
There are exceptions to these rules (e.g. handske) and some of this may also vary with dialect (at least I was told so...)
There are some good links in the overview thread: www.duolingo.com/comment/5892805
Difference betwedn forest and woods in swedish? They are different in terms of size.
I wouldn't say there is a distinction in Swedish. Swedish doesn't really have separate words for bigger or smaller forests as far as I can think. It's just "skog" no matter the size.