Not a teacher
welcome to this forum
Yes, you can use them in both present and past tense.
This condition tells about a situation that is likely to be if you(the person) want- If there is bed here, that will be good.
- If he sees it, he won't believe it.
But, If there is bed here, that will be good. it is not a correct sentence(not sure). Probably, it would be If there was a bed here, that would be good.
This condition tells about a situation that would be unlikely in the future or untrue now- If there was a bed here, that would be good.( actually, there is no bed now as it is untrue now.)
- If he saw it, he wouldn't believe it.(Actually he does not see it . He may see in the future but possibility is very low as it is untrue now.
- For Teachers