Est-ce qu(e) is such a convenient linguistic device for turning statements into questions in French. All that one does is place it before the subject when the sentence is in the standard arrangement of declarative sentences (i.e. Subject + Verb + Rest of the sentence). However, speakers usually use inversion.
First of all, with reference to the writing of your question, please note that the e of the word que should be eliminated. The French use elision rather than have vowel hitting vowel, perhaps, in order that the language remain mellifluous. At any rate, the question should be written as Est-ce qu'elle nage?
[Translation: Does she swim? or Does she go swimming?/ Is she swimming--No helping verbs in French present tense. They are understood]
**Now, to use inversion, the method is fairly easy: simply reverse the order of the subject and verb, putting the verb nage before the subject pronoun elle [Verb + Subject--again use only one verb in French present tense, basic form]. Thus, the question is written in this way:
Nage-t-elle? [Does she swim/ Is she swimming?] Note that a t is inserted in order to prevent the two e's from coming together.
-t- inserted with a double hyphen between a verb ending with a vowel and the pronouns elle, il, or on, is called “t ephelcystic,” as, aime-t-il, dire-t-on.
This insertion of a t only occurs with the third person singular of the -er verbs because the subjects "he" and "she," il and elle, begin with vowels, whereas with the other subject pronouns, when the verb is inverted these pronouns all begin with consonants, so the sound is fluid already without any help.