It's because English grammar is either obvious and doesn't need teaching or is inexplicable and isn't worth trying to make a pattern out of.
I ran, she ran, he ran, they ran, we ran, y'all ran.
I run, she runs, he runs, they run, we run, y'all run.
I'll run, she'll run, he'll run, they'll run, we'll run, y'all'll run.
I thought I'd run, he thought he'd run, they thought they'd run.
We'll have wished we ran, they'll have wished they ran, he'll have wished he ran.
What exactly do you want to teach here? The only weirdness to the rule is that he and she runs with an "s", everything else is the same.
I love you. You love me. They love him. He loves them.
I want you. You want me. They want him. He wants them.
In Russian the above is far different because the word order can be switched for emphasis.
The only thing in English worth teaching is "I have" vs "I am" and almost every language has that same problem as well (Je suis vs J'ai in French).
The hard part with English is that our spelling makes zero sense, unlike Russian or French. The grammar is really quite straightforward.