Translation:You can be really slow sometimes.
Grammatically correct, but slightly different meaning.
"Heel" modifies "langzaam", so you are describing how slow they are. If you translate "heel" as "very", you wouldn't be able to say "You very can be slow sometimes"-- it doesn't make any sense.
"You really can be slow sometimes" -- in this sentence, the meaning of "really" is closer to "actually", because it modifies the verb "to be". As in-- I didn't believe you could be slow, but you really, actually, truly are!!
That being said, either one works well as a complaint about someone's slowness, though I think it'd be more common to say "You really are slow sometimes."
After sorting out the structure of the sentence, it becomes much clearer to me than it does in the first place.
- Je bent langzaam
- Je kunt (soms) langzaam zijn - indicating that you can be slow sometimes
- Je kunt soms heel langzaam zijn- suggesting how slow you can be sometimes.
Meanwhile, I also have a question, is 'Je kunt langzaam zijn' used by native speaker, or it's just a grammatically right but never used sentence /
Well, you could use it as a sentence in real life, mainly the third sentence, to someone who irritates you, although I would use the more common alternative for 'langzaam' which is 'traag'. But I reckon you could only use the first and second sentence if you're talking to a snail.
Well... took the place for explanatory comments out of the choices on "report a problem"... guess they were getting too much flack. That said, I still think "You can really be"... and "You can be really" are saying essentially the same thing. They DO split hairs too much on some of these.
In school we all learnt a group of words is a sentence compound (zinsdeel), if you're able to move the word or group of words to the front of the sentence. So there are many possible word orders possible, but only some are standard, others will be mainly used to put the emphasis on the sentence compound you put first.
Sentence division Jullie/ kunnen/ soms/ heel langzaam/ zijn. ('zijn' is no indepent sentence compound, complementing 'kunnen'.)
The conjugated verb is in second place unless in the question in main clauses (in subclauses you would pile up all the verbs including the conjugated one at the end, eg: dat jullie heel langzaam kunnen zijn). In the main clause the conjugated verb and the subject cannot be separated. All this gives you the following word orders.
Jullie kunnen soms heel langzaam zijn.
Soms kunnen jullie heel langzaam zijn.
Kunnen jullie soms heel langzaam zijn? (yes/no-question)
Heel langzaam kunnen jullie soms zijn. (uncommon alternative, strong emphasis on the first sentence compound)
You can also put this specific adverb after a comma at the end of the sentence in all the cases above. eg. Je kan heel langzaam zijn, soms. I think that that about covers it. :-)