Seules phrasPhrases seules

Sorry, I can't see a meaning for your sentence :)

Did you mean "lonely sentences" as in isolated, disjoint, not correlated ?
"Phrases solitaires"
"Phrases isolées"
"Quelques phrases isolées"
Or let's go for a neologism : "Soliphrases" (just kidding, please don't go there)

Ce livre est léger, mais cette pomme est plus légèrer que le livre.

I dare you to eat the book though... unless you are a bookworm :)

Le flot est grand comme tout un monde.

The words exist, the syntax is perfect, but the semantic is missing... What did you want to say here ?

Je ne sais pas quoice que tu dis !

- Mind the fact that colon, exclamation and interrogation marks are always preceded by a space.
- meaning wise :
"Je ne sais pas ce que tu dis" : I don't know what you say.... weird... unless you have wax and banana in your ears, and then you indeed cannot know, but even like that we would say "je n'entends pas ce que tu dis"
"je ne comprends pas ce que tu dis" : I don't understand what you say, most probably closer to what you wanted to say

C'est quoi le prix?

Right, but a bit familiar. More neutral : "Quel est le prix ?"

Quel est le prix?

Here it is :)

Qu'est-ce que c'i est arrivé ?

- a space before "?"
- "what happened"

Dont livre est ce?

I don't have semantic, and not even sure for the syntax :)
"Is this your book ?" : "est-ce ton livre ?"