Only (1) is defined by grammar. 2,3, and 4 depend on the situation. In 1, you can look up 'present conditional'. Because of the conditional, and the present tense of the first part, you use present tense for the second part. In all of 2,3,4 the choice of tense depends on the timing. You could use any of the alternatives you mention - they just mean different things. e.g. (3)Could you tell me why i should use the past tense , not present tense in this sentence ? She looked very beautiful in the red dress. Note that the 'the' was missing. 'her' would also be OK. This is referring to something that happened in the past. We know that because the writer used past tense. You could write "She looks very beautiful in her red dress'. That means that she is wearing it now. If you wrote this in Chinese, my understanding is that you would normally explicitly mention a time first, so you know when the sentence is set. In English, the tense tells us that.