To strengthen my body, I decided to walk and jog in the park every single morning. To strengthen my body, I decided to walk and jog in the park every single morning.

To strengthen my body, I've decided to walk and jog in the park every single morning. To strengthen my body, I've decided to walk and jog in the park every single morning.

The park is very small and near by my house. The park is very small and near my house.

I think "nearby my house" might be correct, but I'm unsure. Regardless if it's grammatical or not, people definitely say it.

It takes me 15min to walk to the park from my house, and walking around the park also takes 15min. It takes me 15min to walk to the park from my house, and walking around the park also takes 15min.

"15min" is fine for writing casually, but you can also write out "15 minutes".

It takes me 15 min to walk to the park from my house, and walking around the park also takes 15 min. It takes me 15 min to walk to the park from my house, and walking around the park also takes 15 min.

I think the (amount of) time is suitable for me. I think the (amount of) time is suitable for me.

Besides this problem,ut the pronoun "it" is alsousage is weird：in the first sentence, "it" refers to the park, but in the last sentence I want to saytalk about weather, and using "it" to refers to weather. But the pronoun "it" usage is weird：in the first sentence, "it" refers to the park, but in the last sentence I want to talk about weather, and using "it" to refers to weather.

That's another reason why semicolon works better in the second sentence.

Besides this problem, the pronoun "it" is also weird：i. In the first sentence, "it" refers to the park, but in the last sentence I want to say about weather, using "it" to refers to weather. Besides this problem, the pronoun "it" is also weird. In the first sentence, "it" refers to the park, but in the last sentence I want to say about weather, using "it" to refers to weather.

Is ithat right? Is that right?

Essentially, yes. In the first sentence, I also added commas because of the subject switch from "I" to "the park".

Is it right? Is it right?

Yes! While normally you'd want to explicitly state a noun before referring to it with "it," it's standard to just state "It's raining" without explicitly mentioning what "it" is. This is actually an instance of a "dummy pronoun." (Learning about dummy pronouns as a native speaker made me hate English LOL, so I wonder what non-native speakers think of them)

Today, I came to the park, and it was different from the previous few days. Today, I came to the park, and it was different from the previous few days.

Today when I came to the park and it was different from the previous few days. Today when I came to the park it was different from the previous few days.

Original is fine, this sounds a little more natural

It was quieter than usual,; maybe because it's drizzling today. It was quieter than usual; maybe because it's drizzling today.

"It was quieter than usual" and "Maybe because it's drizzling today" can be written as two separate sentences, so it's better to join them with a semicolon.

It was quieter than usual, maybe because it's been drizzling today. It was quieter than usual, maybe because it's been drizzling today.

( I feel [like] the sentences from "Today I came to the park" to "it's drizzling today" sound weird. ( I feel [like] the sentences from "Today I came to the park" to "it's drizzling today" sound weird.

( I feel like the sentences from "Today I came to the park" to "it's drizzling today" sound weird. ( I feel like the sentences from "Today I came to the park" to "it's drizzling today" sound weird.

I think they can be correctjoined into a long sentence, notinstead of written separately. I think they can be joined into a long sentence, instead of written separately.

I think the resulting sentence would feel like a run-on sentence. The way you wrote it is fine.

I think they can be corrected to a long sentence, notinstead of being separately. I think they can be corrected to a long sentence, instead of being separate.

Personally I think they're better as separate sentences. The only real way I see to join them is a semicolon or a dash.