1. It smells where the sun doesn't shine.
(What does 'it' refers to in this sentence?)
2. Open your mouth wide.
3. Open your mouth widely.
(Which one is grammatical?)
4. You should weigh your words before speaking.
(What can we use instead of 'weigh'?)
5. These candles come in many different colors and scents.
(What can we use instead of 'come in' in this sentence? The similar expressions, please.)
1. Nearly all pronouns have antecedents, but in sentences referring to weather in some way (as in this sentence), there is usually no antecedent for "it" --
It's hot outside.
It's snowing!
It's like a sauna outside -- so hot and humid.
http://grammar.ccc.commnet.edu/grammar/pronouns1.htm
3 is grammatically correct, but 2 is used more often.
4. Instead of "weigh" you can use "consider" or "think carefully about" (among a few others).
5. Instead of "come in" you can use "are available in" -- I can't think of any others right off the top of my head!