Hi Siddharth!
I want to add a little to Matthew's wonderful answer.
To your question, let's look at both cases.
CASE 1: "Exam was very hard, there were some critical questions asked and i couldn't answer them" (or) "Exam was very hard, there are some critical questions asked and i couldn't answer them."
In this case, because you are describing something that took place entirely in the past, you would use the past tense for every verb. To use "are" would suggest the action is ongoing or is a general statement (something to be clarified in case 2). Because the test happened and is over, there were some critical questions.
CASE 2: Mom i went to john's house his house was very far from city, his parents were very friendly with me and john had a sister too (or)
Mom i went to john's house his house is very far from city his parents are very friendly with me and john has a sister too.
Let's break down each tense usage.
Mom i went to john's house. his house was/is very far from city.
Because his house is always far from the city, you could use "is". Even now, it still is far from the city. That being said, in everyday speech, the "was" could be referring to your impression of how far his house was from the city (as in: wow, it was really far!). Because you're telling a story about a specific event--going to John's house--, using "was" can be all right!
Similarly, "his parents were/are very friendly." If you want to emphasize the individual memory of his parents, you would use were. As in: when you met them, they were friendly. If you want to say that his parents are generally friendly, you would use "are."
Finally, in John had/has a sister, I would lean towards using "has". Saying John "had" a sister implies she may no longer be with us. So, if she's alive, then "has" is better.
But these usages all come down to a matter of intention and context. If you are describing a specific even that occurred in the past, then use the past tense. If you are describing a general statement that is seemingly unchanging, then you can use the present tense.
So the usages depend on context and what you are trying to communicate. Because you are telling your mom a story, using entirely the past tense can be totally correct. And using the same tense throughout a story is usually a clearer way of speaking/writing. But if you want to communicate things that are seemingly always the case, don't be afraid of switching to the present (like saying he has a sister -- because saying he "had" can be more confusing!).
I know these answers may not seem definitive, but I hope you have a better understanding of what it means to use each tense when.
Siddharth B.
Hey Alex! thanks for an in-depth explanation and breakdown of my examples it helped me :)12/26/19