Translation:We are buying clothes tomorrow in the afternoon.
You cannot use present simple tense to describe one-time actions happening in the future. For that you need present continuos or one of the future tenses (future simple and future continuos makes the most sense).
For this sentence the closest English equivalent would be in present continuous.
I'm sorry but what you're saying is not actually correct. You can totally say something like 'Tomorrow we buy clothes', when you're proclaiming that you're gonna do that tomorrow. You could also say 'Tomorrow we're buying clothes' in that case, but then it's more of a description of what you'll do tomorrow, not a proclamation. Personally I would just say 'Tomorrow we will buy clothes', but I get that that's not the actual meaning of the sentence in Polish. But regardless of your preference, 'Tomorrow we buy clothes' is definitely correct. And it's weird cause this same mistake occurs in other sentences in this series of exercises as well.
Because the Polish sentence is in Present Tense as well. We keep Present Continuous in the future meaning and normal Future Simple separately. After all, if you know some English, you will recognize Future Tense immediately - it has 'will'. In Polish, for a learner, it's not that obvious at first sight that "kupujemy" is Present Tense and "kupimy" is Future Tense.
Besides, "Jutro po południu kupimy ubrania" would sound a bit strange to me, it really would have to be "the clothes" then. Not that this sentence is so great, I'd probably just say "Jutro po południu jedziemy na zakupy" (Tomorrow afternoon we're going shopping) and the fact that we're going to buy clothes is probably known from the context of the conversation.
"Jutro po południu jedziemy na zakupy", ""Jutro po południu idziemy na zakupy", "Jutro po południu jedziemy kupić ubrania", "Jutro po południu pójdziemy kupić ubrania", etc. are sentences I've heard a lot, but the example sentence here is something I've heard for the first time, and the English translation is 100% wrong, because the standard rule of English language is that you cannot combine future adverbs (e.g. tomorrow) with present tense. (see exception at the botttom)
When it comes to the meaning of "Jutro po południu kupujemy ubrania", in English it would mean the same as "Tomorrow afternoon we'll be buying clothes." English language uses future tense with the adverb "tomorrow".
On the other hand, "Jutro po południu kupimy ubrania." would be "Tomorrow afternoon we're going to buy clothes.", no need for the article "the" because the speaker doesn't necessarily know what exactly they want to buy. Also the idiomatic phrase "to be going to" is a future tense (or to be more precise, it's called near future tense), even though it looks like a present tense it's not present tense, and it's one of those things that are confusing about English.
So getting to the point, I suggest fixing either the English sentence to future tense, or changing the Polish sentence to something else.
*Regarding the exception: sometimes simple present can be used, when speaking about the future, as long as it's a fact. For example, when you look at store hours, you can say "The store opens at 10:00 on Saturday." or when you look at a train schedule, then you can say: "The train leaves tomorrow at 9:00." However, if you don't have a definite confirmation of that future event, then you cannot use simple present.
As "we" discussed the plan to go and buy clothes tomorrow, that looks pretty much like a future arrangement to me.
Ok, I'll give it to you that if it's an arrangement between two people, then this translation would work. The way I saw the sentence at the first glance was that it was something said by a mom with children, who decided by herself for the whole family, in which case future tense would be used.
Still, you should allow future tense as well as present continuous, because as you pointed out there's a difference in meaning: future tense means that the action will take place in the future, while present continuous implies that some preparations/arrangements are already made, and the action is already somewhat in progress.
To me it feels that Polish sentence has both meanings.
I see your point, but we have a reason for what we accept and we are not planning to change it.
In English, if only you learned it just a bit, you immediately know that "We buy" and "We are buying" are Present Tenses and "We will buy" is Future Tense. It's just obvious, If you learn Polish but haven't encountered the word for 'buying' yet, you don't really see the difference between "kupujemy" (we are buying/we buy) and "kupimy" (we will buy). They really look similar for a learner. That's the reason we don't accept mixing Present and Future, even if the meaning is virtually the same.
The English translation doesnt make sense. The grammatical differences mean you don't describe something like buying in the present tense when it takes place in the future. Although the lesson is about present tense, the engkish translation would need to be "we will be buying clothes tomorrow afternoon".
And we keep Future to Future and Present to Present. Sure, the meaning of "We will be buying" is almost the same here, but we want "Będziemy kupować" for that.
Ok actually this is an interesting one where English is weird - it would be natural to say "we're buying a car tomorrow afternoon" but not "we're buying clothes..." "We will be" is far more natural in English, but interestingly the other way round for a car. However, you could fix this easily... Just adding "new" or another adjective makes the example natural in English and would stop people like me from whingeing