1) I expect so.

我料想是这样。
3) I guess it is.

我猜是这样。
4) I presume (that) this is true.

我想这是真的。
5) I think this is a fake diamond.

我想这是假钻石。
6) Yes, it is, as far as I know.

就我所知是这样。
参考词条