When do I use 着 to indicate action in progress?
This is used for both actions in progress and things that are happening in present tense, such as "she is wearing a dress".

I sometimes equate this to the -ing suffix.

Examples:


  他穿着西装弹吉他


He (She) is wearing a suit and playing guitar


  你帮我看着他


Help me look out for him (her) (at this moment)


  你看他扛着那么大的一个东西


See him carrying such a big thing

However, this is not used as -ing with actions as you would expect:

This is wrong:


  他在游泳着


The 着 here should be applied to the 游 i.e.


  他在游着


Wrong:


  他在弹吉他着


Right:


  他在弹着吉他