I cut an onion is same in simple present and past present how can we notice that the given sentence is in past or present and also transform it into all tenses

In future tense, we add 'will' and in past we say 'had'. However, 'cut' is a verb which has the same form in infinitive, present tense and past participle. 

You have to read the sentence prior to the sentence, succeeding the sentence or the entire context under which the sentence is being spoken or written to understand the proper tense of the sentence. 

Happy to help!