Damian has done an excellent job with his explanation, concerning the present experience that you are having. To take it a step further, I want you to know that tongues is a language, a manner of speaking. Language is a craft. The nature of a craft is that the more you practise it, it is the more proficient you get at it. Do that and you will find that eventually more words and expressions will be added to it. Notice a child, at first, uses the same word for almost everything, but as she grows older, more vocabulary is added to her treasury of words. So, is it with tongues. So, go ahead, worship the Lord in tongues. Keep repeating what comes to your spirit until you begin to flow like a river. I prophesy that you will begin to flow prohetically in the gift of interpretation of tongues. I can feel it! Say Lord I receive it, right now!