"I guess" is an idiomatic expression that is commonly used by native English speakers and its use is quite distinct from the present simple of "guess" - "I guess..."
It is used when want to express that you believe something is true or likely but are not certain.
In general, "I guess" is very similar to "I suppose," and they are often used interchangeably.
1. To express uncertainty.
"Do you think we'll make it to the finals?"
You can also use this to express confusion.
"I guess he was trying to help, but I have no idea what he was doing."
2. To express reluctance.
"I guess I should start cleaning the garage."
"I guess I'll get back to my homework."
There are a few others ways this can be used.
- To express reluctant consent.
"Didn't you tell me you were going to clean your room?"
- To express reluctant agreement.
This can either mean that you agree with another person but not wholeheartedly, or you actually disagree but you don't want to argue.
Whether it means one or the other is not really clear unless the conversation or discussion continues. Either way, the outcome of the example is the same.
"Our manager is always taking the credit for my work."
3. To appear polite and non-confrontational when stating something.
"I guess we'll be working together from now on."
"I guess you and Eve are dating again."
Here, using "I guess" can imply uncertainty, reluctance or politeness or a combination of all of them.
4. To express disdain.
"I guess she wasn't worth all that attention after all."
5. To express irony and absurdity of a situation
"I guess murder is cheaper than divorce."
6. To express sarcasm.
"I guess spending time with your child is not a priority to you."
7. To express apathy.
"I guess I'll work harder next time."