If you’re looking to learn Sanskrit mantra, I highly recommend studying with someone who reads and writes in Devanagari (the script of Sanskrit). Because the letter forms in Devanagari are different for different tongue placements and aspirated/non-aspirated letters, it would indicate that the teacher is making the distinctions.
At the least, you’ll want to learn from someone who uses diacritical marks in the transliteration system when teaching (dots under and above certain consonants, and dashes above certain vowels, as in the example above). If you’re not sure which tongue position the letter is in, ask!
(Please note, I’m immensely grateful to my mantra teachers along the way, including those who didn’t know Sanskrit themselves. In the final analysis, devotional always trumps skill.)
For instance, there are 4 different “T” letters in Sanskrit (2 different tongue placements + 2 aspirated sound variations = 4). If you don’t know the alphabet, you won’t know to listen for the distinction, and the tendency will be to group them all into the single “T” sounds, and the result is not as precise. (I live in Hawaii, and Hawaiian language only has 13 letters. In Hawaiian, they say “Mele Kalikimaka” on December 25th, because they don’t have all of the letters to say “Merry Christmas.” A similar thing happens to the mantras when they go from Sanskrit to English.)
If you’ve read this far, you most likely have a sincere desire to learn, so I offer this encouragement so you don’t have to spend as long as I did floundering around! I practiced mantras I learned in India for 15 years (without learning the alphabet first), then in 2008 memorized hundreds of mantras as part of an American-published teacher training course, and had started teaching mantras in 2009, before I realized I needed to relearn everything in more detail if I wanted to pursue the precision that my definition of mastery demands. (I’m still a LONG way from that mastery, but at least now I know what I don’t know!) My first 15 years of mantra practice was still immensely beneficial, fueled by sincere intention, but I wish I had taken a bit more time to learn basic skills from the beginning. I had the idea that learning Sanskrit was too hard, but now I know that it’s not. You can do it, too!!
I’ve lived in India for only about 10 years of my life, but that has been long enough to learn that many Indians don’t chant mantras in Sanskrit, either! (Often, they’re Hindi-ized versions, or inflected by another of the 97 Indian languages.) Remember that just because it’s Indian, doesn’t mean it’s Sanskrit!
Really learning the language with its complex grammar would take many years, but for chanting and mantra meditation, at least a basic understanding of the Sanskrit alphabet and the tongue placements will suffice.