for past tense, you can simply add "laew" at the end of the sentence i've been there, i went there = chan pai tee naan laew chan = i pai = went, have been tee naan = there laew = already he was dead, he died = Kao tai laew Kao = he died = tai laew = already for future tense, u just add "Ja" after the subject i'll go there, i'm going there = chan ja pai tee naan chan = i ja = will pai = go tee naan = there I'll call him, i'm going to call him = Chan ja toe ha kao chan = i ja = will toe ha = call kao = him for example ;)

Moreover, we may add some suffix to specify the situation. - แล้ว (already did something): ฉันกินข้าวเช้าแล้ว = I had breakfast already. Otherwise we usually use - เคย as a word used to: ฉันเคยทำงานที่นั่นมาก่อน = I used to work there. For me, I have no idea about the future tense rather than the guy above said. May be just will and tomorrow.

In Thai language, we have no different between past and future tenses. just like Chinese language have no past and future tense. but we can put some words into sentences to make the time obvious, such as 1) เมื่อก่อน (in past) 2) จะ (will, plan) 3) เมื่อวาน (yesterday 4) พรุ่งนี้ (tomorrow)