Hi Quin Vu : ) You would say "I don't know when to use the past tense". 'When to use' refers to something in general, not related to yesterday or today or tomorrow. It could be at any time, so you use the simple present. You can find a good explanation about all the tenses as well as exercises with answers on this nice website: http://www.perfect-english-grammar.com/verb-tenses.html Read the explanation for each tense, and then do the exercises : ) If you need any extra help I am a tutor, feel free to book a session. Also keep asking questions here, many people are willing to help : ) Have a wonderful day : )

I'm not a native speaker, so it's difficult to explain accurately. But at first you should learn past tense auxillary verb DID. Also you should know that verbs in past tense change, you need to add -ed to the end of the word, if it's not an irregular verb. Learn irregular verbs and use them in past tenses. So if you ask a question about past use it. If you want to use a verb TO BE, you have to use its past form WAS or WERE

you should get a grammar textbook and practise a lot