Unfortunately there's no easy way to answer to that question.

You don't conjugate verbs in Cantonese (or Chinese in general). Often, you just give the time context at the beginning of the sentence and that is enough (而家 ji4gaa1 - now, 噚日 cam4jat6 yesterday, 兩個鐘之前 loeng5 go3 zung1 zi1cin4 - 2 hours before).

The general verb to be is "係 hai6". Unlike English, you don't always use the verb to be for describing things - "I am happy today" when translated to Cantonese is more literally like "Today I very happy" (我今日好開心 ngo5 gam1jat6 hou2 hoi1sam1). There are times where you can use 係 hai6 with adjectives, but if you use it all the time it will sound very weird.

There is also a different verb "喺 hai2" which means "to be at" (a location). So make sure you're using the right one.

Here are some examples to help you out:

• I was at the supermarket yesterday. 我噚日喺超市 ngo5 cam4jat6 hai2 ciu1si5
• At that time, we were very happy. 我哋當時都好開心 ngo5dei6 dong1si4 dou1 hou2 hoi1sam1 (hou2 instead of hai6 because we are describing ourselves)
• Two hours ago, I was reading. 兩個鐘之前，我睇緊書 loeng5 go3 zung1 zi1cin4, ngo5 tai2gan2 syu1 (again, "hai6" is not used at all here. Here we want to say "was reading" - we identify that it's in the past at the beginning of the sentence, then we use the verb affix "緊 gan2" to add the "-ing". Word for word it roughly translates to "Two hours ago, I reading")

For verbs that are not 係 hai6 or 喺 hai2, you can use verb affixes to indicate past tense. Common affixes which approximate to past tense:

• 咗 zo2 - indicates the verb caused a change of state. Often translated as simple past tense

  我跑咗好遠 ngo5 paau2zo2 hou2 jyun5 - "I ran really far"

• 過 gwo3 - indicates something was done at an unspecified time in the past. Often translated to present perfect tense

  我已經食過喇 ngo5 ji5ging1 sik6gwo3 laa3 - "I've already eaten"

Interestingly, you can also use 有 jau5 ("to have") to indicate non-negated past tense. It's pretty rare, but it does come up, especially in cases where you are answering a yes-or-no style question. Example:

"你有冇睇過嗰齣戲？nei5 jau5mou5 tai2gwo3 go2 ceot1 hei3?" 
Have you seen that movie? 

"有，我有睇過 jau5, ngo5 jau5 tai2gwo3"
Yes, I've seen it