the verbs need to be in same tense. in other words if, you're talking about the past, you need to *stay* in the past. For example you'd have to say "Sandy gave me a smile while we *were* greeting (the" we're" in your sentence would be read as "we are). additionally, it should be" she must have a crush on me". When "has" is used with other verbs such as "must", "should" or "will", you use "have" instead. Please let me know if you have any corrections or clarifications about my response.