All that KateG says is though more or less accurate, as Murray so deftly pointed out-completely unnecessary. The reason simply is that the folks who talk about all that modal stuff know nothing about music or modes, so when you delve into the matter, you will be completely confused by the disparity--

There are a number of threads, primarily "Modes for Mudcatters"(which, though not currently active, can be dredged up at any time) that beat this issue to death--if Murray's advice seems like it is not enough--simply memorize the melody and then pick it out in your DADGAD tuning--once you can play the tune, you won't care about the modal stuff--

And what key do you suppose you are in when three of the notes you are playing are D?