I agree with the previous answers. Russian language does have modal verbs, such as буду/будешь/etc. and хочу, могу, должен, обязан, надо, можно (надо и можно - adverbs) etc. But there are some differences. The most important difference in comparison to the English modal verbs is that we don't use the position of a modal verb in a sentence to show if it's a question or a statement as it is done in English.
You can swim. -> Can you swim?
Ты можешь плавать. -> Ты можешь плавать?
As you see we don't change the word order. We just use the intonation in this case to show that it's a question. The word order in Russian is flexible (although not arbitrary, there is a logic behind it), and it has different functions.
To show different modalities we also use a mood of the verb (the indicative, imperative and subjunctive moods as well as an independent infinitive)
Hope it helps!