It is extremely hot in the daytime during summer, so it i's good to do something in the early morning.

'It's' is the informal version of 'it is', and sounds more natural when talking about casual things.

I left atround 4:30 AMin the morning and got home atround 6:30 AM.

The sentence was perfectly correct I just wanted to show you a more natural way of saying the same thing. Even if you left at exactly 4:30 AM and returned at exactly 6:30 AM, we still use 'around' when referring to common time increments (on the hour, half hour, or quarter hour). So you might say 'around 6:45' but not 'We'll plan for around 6:53.'

AM and PM are reserved for when giving very precise time and bring with them a sense of importance. You'll typically hear them only when in needs to be specifically clarified if it is AM or PM, when announcing bus or plane times, or when someone is making a pronouncement.

Otherwise, we will use 'in the morning,' or 'in the afternoon, or 'in the evening,' or at 'at night,' depending on the time instead of AM or PM.

There is a pass has one of the steepest slopes in Japan.

'One' is already a singular object, so it doesn't need to be transformed into a singular object by 'a.'

This thought is incomplete: "There is one of the steepest slopes in japan at Kusagari Pass." But because the previous sentence already tells us we're at Kusagari pass it sounds weird to say it again as a location. So I reordered the sentence so it is now a description of Kuragari Pass's properties, rather than a description of a location in japan.

It felt good because it was early in the morning.

That is correct but this might sound a little more natural.

'Early in the morning', 'In the middle of the afternoon', 'Late in the evening,' are some good stock phrases to use in regards to time.