I don't understand it altogether.

[birdeen's call]

Does this sentence mean "I don't understand a single thing of it" or "there are some things in it that I don't understand"?

In Polish it depends on the placement of "altogether" but I think this isn't the case in English (also I'm not sure if "I don't altogether understand it" is a correct English sentence).

[euncu]

***neither a teacher nor a native-speaker***

Maybe, there are several aspects of a matter, they separately understandable but when you put all them together they seem not to make sense.

[kfredson]

Does this sentence mean "I don't understand a single thing of it" or "there are some things in it that I don't understand"?

In Polish it depends on the placement of "altogether" but I think this isn't the case in English (also I'm not sure if "I don't altogether understand it" is a correct English sentence).

Thank you for bringing this challenging sentence.

I agree with Gillnetter that it could mean that I understand some but not all of it. However, the sentence is somewhat ambiguous.

In general, If I wished to convey this idea of partial understanding, I would say,
"I don't altogether understand it." Then it would be crystal clear that some of it, at least, has been understood.

If, on the other hand, I wanted to say that I hadn't understood it at all, I would tend to write it in the way you have written it here,
"I don't understand it altogether." It would be ambiguous, however, so it would be well to rephrase it.

If I were speaking, this could be made clear by the accent I place on the words. Thus,
"I don't understand it altogether" would mean something different from "I don't understand it altogether."
To my mind the first version conveys partial understanding, while the second indicates a total lack of comprehension.

I would be interested in getting the responses of others on this interesting subject.

[birdeen's call]

Thank you for bringing this challenging sentence.

I agree with Gillnetter that it could mean that I understand some but not all of it. However, the sentence is somewhat ambiguous.

In general, If I wished to convey this idea of partial understanding, I would say,
"I don't altogether understand it." Then it would be crystal clear that some of it, at least, has been understood.

If, on the other hand, I wanted to say that I hadn't understood it at all, I would tend to write it in the way you have written it here,
"I don't understand it altogether." It would be ambiguous, however, so it would be well to rephrase it.

If I were speaking, this could be made clear by the accent I place on the words. Thus,
"I don't understand it altogether" would mean something different from "I don't understand it altogether."
To my mind the first version conveys partial understanding, while the second indicates a total lack of comprehension.

I would be interested in getting the responses of others on this interesting subject.

So it actually seems it's just like in Polish Thank you, I suspected it!