Can I use trotzdem instead of obwohl? For example:
Trotzdem er krank war, ging er noch zur Arbeit.
Obwohl er krank war, ging er noch zur Arbeit.
Agreeing with thekeyofgb's comment, it is not apparent that they are similar. But in used in their right place, they convey the same effect:
Er war krank. Trotzdem ging er zur Arbeit.
Obwohl er krank war, ging er zur Arbeit.
There is a third (at least) possibility trotz+(
Dativ is used in Switzerland more often)
Trotz seiner Krankheit ging er zur Arbeit.
Er ging trotz seiner Krankheit zur Arbeit.
According to wiktionary the order you put the sentence in, is also allowed, but it is umgangssprachlich:
Trotzdem er krank war, ging er zur Arbeit. (umg. and pronounced [tʀɔʦˈdeːm])
I must say that I've never heard the trotzdem in this subjunctional use. The pronunciation remark, added by Emanuel, is that the second syllable should be stressed if trotzdem is intended with this meaning (opposite to [ˈtʀɔʦdeːm], the adverb.)
In my experience, trotzdem is used far less as a conjunction and the alternatives using the adverb or the preposition trotz seem more standard/usual. I looked this up only because I am being asked to correct a translation from a slightly bastardised English "Yesterday my school day was wonderful, although we had a test in physics." We wouldn't necessarily say although in English here, it sounds awkward. If you use obwohl for although, it is like a return serve, taking something away from the wonderful day. If you use trotzdem it sounds as if any negative impact of the test was overcome and of little consequence. I am not sure what the English intends, but although sounds somewhat awkward to me. I agree with the above where it states the obwohl clause ususally comes first, and in this case it renders the meaning of trotzdem! Gestern war mein Schultag toll, obwohl wir in Physik eine Klassenarbeit geschrieben haben. Obwohl wir in Physik eine Klassenarbeit geschrieben haben, war mein Schultag gestern toll. Gestern war mein Schultag toll, trotzdem wir eine Klassenarbeit geschrieben haben. Have a think.
These are two different words. "Trotzdem" means "nevertheless," while "obwohl" means "although."
Surprisingly, they can be used somewhat "interchanageably," because they are complementary.
The basic sentence would be: "Obwohl er krank war, ging er trotzdem noch zur Arbeit." Although he was sick, he nevertheless went to work.
Two similar example sentences can then be formed by omitting one or the other, which shows their complementarity, by "interchanging" them.
Obwohl er krank war, ging er noch zur Arbeit. (Omit trotzdem.)
Er war krank; trotzdem ging er noch zur Arbeit. (Omit obwohl.)