"A piece of string"
Translation:En bit snöre
In Swedish, strings are countable, ett snöre, två snören. But in English, they don't really use a string that way. So ett snöre in Swedish would normally be 'a piece of string' in English.
But it is confusing. 'Ett snöre' I rather translate as 'one (whole piece of) string'. But 'a piece of string' is, I think, when I cut e.g. an inch out of it. On the other hand, 'one piece of string' again refers to the whole string. Maybe I'm just overthink the sentence. English is not my native.
We can say "a string". "Do you have a string for this kite?" It depends on whether the person has a string or comes back with a big roll of string that you will need to cut a piece from.
by once reviewing, I got this sentence for more than 10th time?! what is wrong here?! come on, there have been more than thousand sentences to review
This is probably nothing to do with the Swedish course specifially, more about how Duo works in general. They run a lot of A/B tests (they let different groups of users have different experiences, and then they check which group got the best results) so that how the lessons work can change considerably. I'm currently getting sentences over and over again until I get them right at least once, but it may work differently for you if you're in a different test group. There are usually many discussions about this in the general forums.