Used to refer to the current topic or item ('as for this' / 'regarding this') — commonly used in speech and writing.
これは大事な書類だから、なくさないでください。
This is an important document, so please don't lose it.
これはどういう意味ですか？
What does this mean?
これは私の考えですが、別の方法もあります。
This is my opinion, but there are other methods as well.
An interjection used to get someone's attention ('hey there', 'I say'), sometimes expressing surprise when encountering something unexpected.
これは！久しぶりだね、元気にしてたか？
Well, well! Long time no see — how have you been?
これは危ない、すぐ離れなさい。
Hey — that's dangerous, get away immediately.