Today, it was quite hot. Today, it was quite hot.

You generally need a comma when adverbial phrases are at the beginning of a sentence.

Today iIt was quite hot today. It was quite hot today.

sounds more natural

When the weather igets too hot, I cannot stay in my room. When the weather gets too hot, I cannot stay in my room.

When the weather is too hot, I canno't stay in my room, so I go outside for a walk. When the weather is too hot, I can't stay in my room, so I go outside for a walk.

SoBecause of that, I go outside for a walk. Because of that, I go outside for a walk.

"So" is fine verbally, but in writing, it makes the sentence sound incomplete. Some people wouldn't use "because" at the beginning of a sentence, but to me it's fine as long as it doesn't result in a sentence fragment. Personally, I might use "will" before "go," but I think it is fine as is.

So, I gowent outside for a walk. So, I went outside for a walk.

Since you mentioned was quite hot, I changed this to past tense

So, I go outside for a walk.

ifWhen it rains, I am usually happy because I can stay in my room and study. When it rains, I am usually happy because I can stay in my room and study.

"If it rains" would be understood, but "when" makes more sense to me.

I am not against hot weather,; it is just that I cannot study when it is hot. I am not against hot weather; it is just that I cannot study when it is hot.

The two clauses are complete in themselves, so punctuation to designate that or a conjunction is needed. There are certain times when it is stylistically appropriate to omit conjunctions and use commas, but generally, those fall into one of these categories: - Have more than two items - Itemized phrase is relatively short.

I am not against hot weather, ibut it's just that I cannot study when it igets hot. I am not against hot weather, but it's just that I cannot study when it gets hot.