1. Verbs ending in MASU normally are pronounced MAS.
2. DESU normally is pronounced DES (this is mentioned in Lesson 1 grammar)
If U is dropped all the time then why even have it in the word??
This just seemed like an obvious follow up question. The answer is because when the words are written in Hiragana they contain the Hiragana symbol that represents the sound SU (す) so this is why it is represented in Hiragana as SU and not just S. There isn't any symbol in Hiragana for S alone so we write it as it should be in Hiragana.
As I mentioned before there are times where the U isn't silent but there is no rule written in any book about it.
Here is an example of a very confusing thing about English that I use in class to show students that sometimes there just isn't a good answer:
Look at the following combination of letters in English: OUGH.
Now look at them in a variety of words. Please read the words out loud.
TOUGH (pronounced TUFF)
THOUGH (pronounced THOW)
THROUGH (pronounced THRU)
The same 4 letters in different words are pronounced differently. How crazy is that?
I also tell students that NO matter how much you study on YesJapan.com or in our class that you will actually learn more Japanese from Japanese people themselves. Lessons are a guide but you won't learn anything unless you go out in the field and make tons of embarrassing mistakes. This is the only way to learn.
There might not be an easy answer to this question but I have noticed some commonalities in the use of the trailing U by japanese speakers; I have noticed when people stress a sentence to a greater degree they will say the U very distinctly. I have heard japanese women (but never japanese men; I don't know why) say "Ah, sou desUU" and arigatou gozaimasUU!
Pronounciation of u and i Even though I am at the early stages of learning Japanese, I agree that it seems to be more of a question of knowing when to pronounce it & when no to...however in a Japanese Grammar book I have, it says:
"Sometimes the i and the u aren't pronounced. This usually occurs between voiceless consonants (p, t, k, ch, f, h, s, sh), or at the end of a word following a voiceless consonant." Ex: sukiyaki = skee-yah-kee
Hope this helps!
Unvoiced U and I The rule I read in my books is that U and I are silent between unvoiced consonants (S, T, H/F, K), or after an unvoiced consonant at the end of a word.
If all the consonants surrounding a vowel are unvoiced, then "I"s and "U"s are silent.
Sou desu (Soo Dess)
Sou desu ga (Soo Desu ga)
In the first phrase, you don't pronounce the "u" at the end of "desu", because all the consonants around it are unvoiced (again, the unvoiced consontants are S, T, H/F, and K). But in the second phrase the U is pronounced, because it has a voiced consonant on one side.
sometimes its said and sometimes not I've noticed a lot of times some speakers say おはよう ございます and pronounce it both with and without the "u". I'm guessing that a lot of the times they added the "u" at the end to give a more "cute" feeling as I've yet to figure any rule in that particular phrase concerning it.
It depends on many factors: Politeness x Speed of Speech x Following Word x Personality x Mood
An english example of what I mean by Speed of Speech: If you're in a hurry, you might say "'Godda/Gotta" instead of "Got to" or "Teller" instead of "Tell her". Or to be even more simple... "Why do you use "he's" instead of "he has"?" It's sort of like the same with the "u" in Japanese. If you're in a hurry, you'd(<- look at all the abbreviations, I did it again), say "Dame des" instead of "Dame desu".
I also noticed some of them use the "u" to add politeness (I even think I've read that in a book...but I'm not sure). It's hard to compare anime with real life, but if you watch it too much like I do, you'll eventually notice that they pronouce the "u" in "desu" when they're following orders, or just talking to someone of a higher status.
Mood: A girl would most likely say: "Zettai dame desuuuu!!" if she was mad. And "Zettai dame desu." if she was serious.
I won't mention the "following word" issue since a bunch of people already have.
And sort of summing it up; It's a lot about personality. So mostly... :) It's your choice.