Common misuse
In American English, the phrase "I guess" frequently does not have the same literal meaning as the verb "to guess", and so is often misused by nonnative English speakers.
One example of this misuse can be found in this question about predicting sports outcomes. The OP wants to express that, though they are uncertain, they believe their team will win the game. In American English, we would generally use "I think", or even "I bet they will win the game," indicating that the speaker feels this outcome is likely, without adding any additional subtext. (Note: "Bet" is used nonliterally, here, and does not imply making an actual wager.)
As discussed below, the phrase "I guess" often implies a subtext of doubt, disagreement, or dissatisfaction, transforming or even reversing the meaning of the sentence. Saying "I guess they will win the game," generally does not mean that you are uncertain about what will occur, but think they might win. Rather, the sentence is focused on its subtext. Depending on the speaker's tone of voice and the context, it might indicate that either:
- speaker is surprised that they will win
- speaker disappointed that they will win
Avoid this common misuse by preferring "I (think/believe/bet/suspect/am almost certain) they will win", and avoiding "I guess" unless you want to use one of its useful subtexts.
Clarifying Common Usage
The verb "to guess" means "to give an answer to a particular question when you do not have all the facts and so cannot be certain if you are correct" (Cambridge) or loosely "to select the correct answer despite uncertainty".
The phrase "I guess", on the other hand, is not generally used in American English to describe the process of guessing. Rather, the phrase "I guess" implies some kind of internal discovery, conflict, or disagreement. This is a challenging idea to grasp, so we'll proceed by example.
Usage 1 - Disagreement:
Context: Laura has never been friendly to John, so John thinks she is mean. John's friend Kate meets Laura for the first time, and has a good first impression of Laura:
Kate: "Laura is so cool!"
John: "Sure, I guess."
Note that John is not literally guessing anything. If spoken aloud, "I guess" might be inflected similarly to the way in which Americans inflect questions, or doubt, with the word guess rising slightly in pitch, or rising and falling again rapidly. In this conversation, John is using "I guess" to politely express doubt or disagreement with Kate's opinion.
If he replied "No, Laura's really mean," that might be perceived as confrontational. Instead, he uses "I guess" to introduce uncertainty into his agreement, producing the effect of a "softened" disagreement. Hearing John's answer, Laura might reply "You don't think so? What did she do to you?"
Usage 2 - Doubt:
Context: Laura has still never been friendly to John, so John thinks she is mean. John's friend Kate tells him that Laura has been helping her with her homework a lot lately.
Kate: "John, you and Laura have a lot in common."
John: "I guess that could be true, but she's always talking about dumb stuff."
Though John is "giving an answer without having all the facts" here, the subtext of his response is critical. By using "I guess", he is not saying that he suspects Kate might be right. To the contrary, he is using "I guess" to imply that he doubts Katie's statement is true. The second phrase in this example clarifies his intention. In the right context, it would be correct for John to say simply "I guess," indicating his doubt that he and Laura have anything in common with his tone of voice.
Usage 3 - Reluctance/Resistance
Context: immediately following the statements in Usage 2.
Kate: "Well, you should at least try to get to know Laura better before you judge her."
John: "I guess so, she just really bugs me."
Here, John is expressing that Katie might be right, while expressing resistance to the idea of getting to know Laura better. As in Usage 2, the second phrase in this example clarifies John's intention. In the right spoken context it would be correct for John to say simply "I guess." With a negative tone of voice, this shorter construction could indicate reluctance/resistance to Kate's suggestion. With a positive tone, it could indicate wholehearted agreement, with a subtext of... discovery!
Usage 4 - discovery:
Context: Laura has never been nice to John, so John thinks she is mean. One day, Kate convinces John and Laura to go for a hike with her. Once they both get over their shyness, they have a really great time. After Laura goes home, John might tell Kate
"Wow! I guess Laura really is a nice person after all!
Note that again, John is not guessing. Rather, John has realized, based on new knowledge, that his prior assessment of Laura may have been incorrect. By using "I guess", John is acknowledging a change in his perspective.
A similar usage is discussed in this question, in which "I guess" is used to indicate a conflict between a currently-held expectation, and a possible schedule change. In this type of usage, "I guess" is used to indicate a significant change - of perspective, of plans, etc.
Some literal exceptions
From another answer
@dwilli's first example shows one case in which "I guess" behaves similarly to "to guess". (Both other great examples in that answer showcase nonliteral uses of "guess").
Though this example is valid Am.English, it may be a little misleading. If I were the speaker, I would probably say this a little differently, for clarity. Because "I guess" is so frequently used to express doubt or conflict, my intention would impact my use.
To intentionally express doubt, confusion, or strong uncertainty that he actually went home, I would use:
Where's George? I guess he went home? He said he was tired, but I don't know because I didn't see him leave.
Or to express my belief that George probably did go home, I would use:
Where's George? I (think/bet/suspect/would guess) he went home, because he said he was tired, but I don't know because I didn't see him leave.
Note that in this construction, I use "would guess", not I guess. "Would guess" functions literally, without the subtextual baggage of "I guess."
Literally guessing
Note: Although the following conversation is correct Am.English, it is a relatively uncommon usage, and tone of voice is important.
Q: "Guess a number between 1 and 10."
A: "I guess 4."
In real life, the words "I guess" would often be omitted. This is more common:
Q: "Guess a number between 1 and 10."
A: "4"
Why? Probably mostly for reasons of simplicity/laziness in spoken language, but this simpler construction also avoids the different and slightly context-inappropriate subtexts of the common phrase "I guess".
