Questions & Answers about Они не могут танцевать быстро.
I know the dictionary form of 'can' is мочь. Why does it change to могут here?
Мочь is the infinitive (dictionary form), but it must change to match the subject Они (they). The verb мочь has an unusual pattern where its core letters change. For 'they', the correct form is могут. Other common forms you will see are я могу (I can) and он может (he can).
Does the verb танцевать need to change its ending to match Они?
No! When you use a modal verb like могут (can) or хотят (want), the second action verb stays in its dictionary form. This is exactly like English, where we say 'they cannot dance', not 'they cannot dances'. That is why танцевать keeps its standard -ть ending here.
Why is не placed before могут instead of before танцевать?
In Russian, the word не (not) always goes immediately before the exact word it negates. Because the sentence means they lack the ability to dance fast, не must go right before могут. If you instead said Они могут не танцевать, the meaning would shift entirely to 'They are able to not dance' (meaning they have the choice to sit out).
Why does the word for 'fast' end in -о (быстро)?