Oh what I would give to have a larger vocabulary! I am the opposite, I have a fair bit of grammar, but vocabulary is rather lacking, since I more enjoy learning why a certain phrase does what is does, over simply learning words etc. There are more consonants and vowles in hindi, however, English joins many of our sounds together to make strange sounds that you cant really study without being expossed to it. Hindi is very simple when it comes to this, they do join different sounds together, however they dont really change, they just drop a little off and add something (so saying dhy instead of dha etc) So once you get the sounds under your belt, its a piece of cake!
At first the dental and retroflux may seem kinda similar, depending on the speaker! Firstly, Dental is really really light, used on your front teeth with your tongue. With the retroflux, your tongue is curled back to the roof of your mouth and is VERY hard. When you use dental, don't put a lot of effort into it, so it is light, but with the retroflux, push your tongue up hard and then make the required sound. You will hear a difference. Just doing this will greatly improve your accent, (In my dialect - Australian English - we don't even really use our tongue, so its a nice change.)
In response to your rahaa and rahee, that is the present continuous tense and the slight difference is only showing a gender and number, aa being singular male and ee being multiples (With AT LEAST one male - if they were all female it would be ii.) However, in reality, this isn't always followed, Ive spoken to people who use whatever, whenever they want!