When you add sugar to it, your cup of coffee becomes not only sweet, also fattening.

which part is wrong??

4 answers

Antecedents should occur in a sentence before any pronouns that refer to them. Which pronoun and antecedent need to be reversed in this sentence.

In addition, that last phrase ("also fattening") seems to be missing some kind of connection to the rest of the sentence. Read it aloud and think about what needs to be added here.
Here are two very good websites to use to help you with grammar and usage, as well as writing:

http://www.chompchomp.com/terms.htm

http://grammar.ccc.commnet.edu/grammar/index2.htm

Bookmark these and use them in addition to your text.
... and one more, for help with both grammar and with writing:

http://teachro.publiccomputingservices.org/

Bookmark it!
also

is wronh one??