Learn English – Clear the stock or Clear out the stock

They are synonyms. Someone may be preferred because it's a syllable shorter (link), but they mean exactly the same.

In this context, the use of the word "think" is generally used to soften the statement.

"I don't understand" and "I don't think I understand" mean basically the same thing, but the first one is more blunt, and in some cases could be interpreted as confrontational or impolite (with an implication of "I don't understand because you failed to explain it properly", for example), while the second form makes a point of focusing the statement on the speaker themselves and their own thoughts, making it seem more about them than about the other person ("I don't understand, but that's probably my fault").

"I don't think I understand" can also be used to express some amount of confusion, instead of just lack of understanding. It can have the sense of "I thought I understood, but then something else made me think my understanding was actually wrong".