You can also use the followings: In my perspective In my opinion

I believe In my opinion It seems to me that I find that (meaning: in my experience I have come to think/believe) I'm pretty sure

Here are a few alternatives, which, as in Spanish, convey essentially the same meaning with some slight nuances. I think we'll be able to deliver the project on time. I expect we'll be able to deliver the project on time. I believe we'll be able to deliver the project on time. I presume we'll be able to deliver the project on time. I imagine we'll be able to deliver the project on time. I envision we'll be able to deliver the project on time.

I believe

Just don't say it all the time :) Or you could use softeners like "perhaps", or "it seems".