"ne <verb> que" is a negation to mean "only." "Je ne mange que des sandwichs" means "I only eat sandwiches". So the negation here is as follows:
"Elle porte une chaussure" = "She wears one shoe"
"Elle ne porte qu'une chaussure" = "She only wears one shoe"
In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). - she is wearing means she currently wears In French, this verbal form does not exist (directly translated “elle est portant” is incorrect). Therefore, you can translate either “elle porte” or “elle est en train de porter”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.
Thanks a lot, I didn't know that. I'm afraid that you might have accidentally posted this on the wrong sentence though.
The English was "she is (only) wearing" which is a continuous case and your question was about the grammar of the sentence. I therefore assumed you would be interested in knowing about the construction of the verbal form. However, if I taught you something, that was worth the error!
There were a couple like that. Perhaps it's so that we'll pick up that a "que" in the middle of the sentence doesn't always mean a subjunctive is used.