I'm Blue Spark and I'm a Magical Engineer and a alicorn, how I got here is a long story but I'm writing it here so you can read it.
(This is my first story and I'm not good at writing so pardon any grammar and spelling mistakes)
(I can't promise any consistency in chapter releases)
(Tags will change as needed)
This is a bit rushed.
I just skimmed over the story real quick, it seems a little bit rushed, I suggest getting an editor you can get an editor by going to the group looking for editors.
Good luck with your story.
There's a lot more detail you could go into - Chapter 0 could easily be an entire story by itself if you wanted it to. Writing isn't merely about describing things that are happening - it's also an opportunity to create an experience for the reader.
These two ponies should be the most important figures in the protagonist's life - his mother in particular is going to be a constant presence for the next few years. She's going to shape how he views this new world, how he understands pony culture. Yet, we know nothing about her. I assume she is a loving mother (not always a given, for humans) - is that a change for the protagonist? Is she the mother he always wanted but never had? You don't have to explain all these things in the story, but they should at least be thought about, as they'll influence how she's written.
Most humans don't remember being babies at all, so this is indeed an interesting opportunity for the protagonist to describe the experience first-hand - assuming he wants to. Babies are not held to the modesty standards of adults and have a very intimate bond with their mothers, so it might actually be a rather delicate topic for him. But even a refusal to discuss the specifics would tell us something about the experience.
Baby teeth don't come in all at once - it's a gradual process which takes a few years to complete for human babies. Usually they get the front ones first.
Also teething is generally uncomfortable for a baby, and in worse cases painful, so good luck experiencing that first-hand. :)
For horses, teething works a little differently; foals can actually develop their first teeth before they're born. It's up to you to decide how teeth work for Equestrian babies.
Bit of a strange line. A baby's first steps and their first words are obviously two separate events, and obviously the parents would be excited for both. Incidentally, real-life horses actually start walking a few hours after birth.
How does he feel about that, though? If it was me I'd find it incredibly draining to have this massive secret and to be essentially living a lie, yet he takes it in stride.
Also it's been more than 1000 days, so he'd have had his first birthday by now. That could be an event in his life.
Actually it's always normal to be four years old when you're that age. I assume you meant that it's normal for Equestrian foals to go to school at the age of four.
You can't say "and stuff" in a story. The point of a story is to describe that stuff - if it's not important, don't mention it.
It's interesting that the protagonist doesn't want to stand out due to his reincarnated experience, but I still don't know why. Is he afraid of being found out? Does he fear it will make it harder for him to relate to other ponies?
He was four years old a moment ago.
Whoa, now he's doubled in age. That escalated quickly.
Not sure I understood this. I'm guessing maybe magic and technology are mutually exclusive domains, and the protagonist found a way to unite the two? Sounds great! So he must be building machines, right? How's he keeping that secret? Why would he want to? Is he using any of his past-life experience to do this, or is this something new for him?
This is very confusingly written. Going from "nervous in the back of his mind" to the sonic rainboom makes it seem like the rainboom is happening in his head, rather than something he's witnessing. Also, remember that a sonic rainboom is a spectacular event that fills the sky - it can't simply be an partial afterthought to him thinking about his cutie mark. Also also, is this meant to imply that the sonic rainboom occurred at the same time he got his cutie mark? Also also also, the way it's written seems like he's witnessing the Mare in the Moon in that moment, which is impossible as the Sonic Rainboom occurred at daytime. Also also also also, it feels a bit late for him to be mentioning the Mare in the Moon now. He's been living as a pony for ten years (close to 30 human years, if days are the same length as Earth days). He'd have been aware of the Mare in the Moon for his entire pony life, and undoubtedly would even have figured out roughly what time period he's living in relative to the show. That feels like something that would have come up a lot earlier.
Just a few things to think about there. :) Keep at it!