[lemoncucumber]

A similar example:

As a reply to the question, “what’s the most expensive car on the market?”

1) “I’m guessing it’s some German supercar.”

2) “I would guess it’s some German supercar.”

3) “I guess it’s some German supercar.”

As a native speaker, the first two both sound correct to me, and essentially equivalent. The third sounds a little weird, even though it’s grammatically the simplest. I can’t put my finger on exactly why.

[rsa4046]

The first two seem consistent as tentative responses to a statement that might contain hyperbole, joke, play on words, double entendre, etc., by maintaining some distance with the statement of supercar-as-fact. In contrast, the third seems to accept the fact outright, and lets the accuracy of the responder assume the uncertainty.

[Talanes]

The third reply strips so much away that the tone changes from tentative to apathetic. The third speaker is just doing the bare minimum to keep conversation moving.

[sharken]

My thought as well, the third gives the impression that the person answering doesn't care much for the question.

[reph2097]

That's exactly what a German speaker would do.

[archduck]

In 1) and 2), it's clear the speaker is making some kind of conjecture. In 3), I think there's some bleed-over interfering from the idiomatic "I guess" which indicates that you're reporting hearsay or hedging your commitment to its accuracy. Some languages express this "evidentiary" modality more formally, e.g. via some kind of morphosyntactic change that makes it explicit. English has a ton of ways of expressing modalities, but they're wrapped up together in various kinds of constructions that express tense and aspect as well ("TAM" is an acronym for tense-aspect-mood used in lots of linguistic analysis across all kinds of languages).

Usually it's an error to add -ing to a mental process (Are you knowing the answer?, I'm believing in a deity, etc.). (We can choose to construe a mental process as a material one - it's not a lexical rule about those verbs, but a grammatical means we have access to, e.g. I'm lovin' it, I'm thinking about you contrue them as an activity rather than a mental state.) But perhaps the reason why replacing "guessing" with "guess" changes the meaning so much is because "I guess" is already taken. It should modify the meaning as "I think x" => "I'm thinking x" does, but saying "I guess" sounds like a way of hedging information from a 3rd party.

[sharken]

Not a native speaker, but 1) and 2) seem equal, while 3) somehow gives the impression that you don't really care about answering the question.

In danish the word "Ligeglad" is the state of mind for 3).

[irrational]

I would say “I’d guess it’s some German supercar.” Does contracting the “I would” make it more difficult for a non native speaker to understand?

[detaro]

As a non-native speaker, I'd say it depends on their background and how they learned. To me it feels completely normal and the non-contracted form took longer for me to parse, I think because the contraction is more common in colloquial communication with native speakers, but I'm also sure that if you tried this on me at a point in life where I had mostly school english to go on, the contraction would be more difficult.