Soren....

[ShepherdsWarrior]

Okay so I've always wondered what Soren meant by he couldn't speak. Did he not know how? If so, who taught him? Ike?

[Augestein]

He meant that he never had anyone to teach him how to talk. It's implied that he learned to talk as he needed to learn to speak to people. 

[Extrasolar]

In his support with Ike he explains that he never learned to speak for a while because first, the woman who cared for him in his youngest days only talked to him to insult him and complain that she was "cursed" with his presence and whatnot, so he never had to learn to speak to answer her.

The old sage that took him in from the woman likely taught him nothing but magic invocations, and after an unknown amount of time (can't be more than a few years) the sage died and Soren took off on his own.

Soren says Ike was the first person who was really kind to him, so I imagine it was Ike that helped him get talking, and to a lesser extent likely Greil, Mist, and the more compassionate and caring of the mercenaries. Basically, I doubt Shinon looked at him twice. Lol.

Edited April 17, 2017 by Extrasolar

[Interdimensional Observer]

As for actually learning how to speak, Soren presumably left for Crimea after Ike and Greil got out of Gallia. As a child vagabond, he was probably scorned by others and lived in poverty, but he forced himself to learn to talk. Then, after acquiring more knowledge and figuring out how to survive, he somehow managed to find the GMs and Greil let the stray fellow in. Why, I'm not sure.

 

20 minutes ago, Extrasolar said:

Basically, I doubt Shinon looked at him twice. Lol.

In case you already hadn't read this:

Spoiler

Shinon vs Soren

Shinon: Well, well, lookee here. The little boy plays soldier… How sweet. Your job’s always been just to use that big brain of yours. That’s why you spend your time hiding behind Ike’s apron. Isn’t it, Soren?

Soren: Shinon. You always were dumb as a gnat, but… A first-class archer. I’ll give you that. But with the power I now possess, it’s probably in your best interest not to presume to know my strength.

Shinon: Hah! You’re still a snotty little whelp, aren’t you? Get ready to die, little man.

 

Shinon's fight convos with all the GMs are rather nice. He's kinda cold but caring towards Gatrie, fairly benign towards Mist (ties in well with his Rolf support), and outright respectful towards Titania.

And going back to Soren, slam dunk vs. Petrine:

Spoiler

Petrine vs Soren

Petrine: You… That mark on your brow… That’s not a charm of the dead, is it? You’re no Spirit Charmer! Hmph! You may be able to fool others with that, but not me. It’s because we’re the same, see?

Soren: …The same? Don’t be ridiculous. I’m nothing like you. You kill for sport, and hide your fear behind a wall of bravado. Now let me show you true fear!

 

[NJ7009]

For the first part of your question, I defer to everyone else in this thread. He just couldn't speak a word of modern tongue although he understood it fine.

However, as for how he learned to talk, Soren actually says how in that special base conversation he has with Ike. He says:

Spoiler

Soren: When I reached Crimea, I took refuge in a church along the way. They took my brand to be a sign that I had been trained in the arcane arts. They took good care of me, and taught me things. Once I had learned to speak and behave like other people, I wandered Crimea for several years. Then I finally found you.