Are these sentences OK? Or, what do you say?
Be careful. There's broken glasss on the floor. / There are glass shards on the floor.

Question

Answer

Both good number 1 is more common. 
number 2 would be used when the glass has broken into obvious shards = a sharp knife like shape or very small piece that can easily cut because they are difficult to see.

basically they are describing two ways in which glass is inclined to break and shatter.

Answer

Those sentences are perfect. If you wanted to, you can add a comma "," after the word careful. Also it was probably a typing error but one less "s" on the word glass. Nice job!

Answer

They're both fine but the first one is more commonly used

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