When using 对 (duì) as a preposition, it is used to indicate "to" or "towards" a object or target. As with all prepositions, some care should be taken when using this particle, as usage of 对 doesn't always totally "make sense" or correspond to English at all in Chinese grammar.

Basic Structure 
The structure is basically:
对+object+verb phrase
Note that you shouldn't be plugging in just any old verb here. Certain verbs are often used with 对, and some even need 对 if you want to add an object (like 感兴趣, explained below).

Examples 

Phrases
对 is also used in such structures as: 对……来说 ("for... [a person]") ,对...感兴趣 ("to be interested in...") as well as 对···的印象（”the impression on the object“）
对……来说:This essentially means "for... (a person)." It's for making a statement from a certain person's point of view.

The pattern is:
对+person+来说

对……感兴趣 :In English was say "to be interested in something." In Chinese, the pattern is:
对+object+感兴趣

对······的印象:This basically means "the impression on what you saw or what you know."
The pattern is:
对+object+的印象