Desu or Des
Different languages have different pronunciations as well as letters.
I think Japanese phonemes written with 46 hiragana are relatively simple. They are sometimes written in the roman alphabet in the textbook to help non-Japanese read them. However, actually I often see this confuses my students. This way of writing Japanese words in the alphabet started when James Curtis Hepburn made a Japanese-English dictionary in 1867. He used this way called “Hepburn Romanization” for Japanese words in this dictionary.
（http://en.wikipedia.org/wiki/Romanization_of_Japanese）
There is another type of Romanization which the Japanese government established based on Hepburn Romanization.
I can roughly say 90% of students ask me one question at the first lesson.
「です」 is not “desu”. You pronounce it as “des”, don’t you? Why do you write “desu”?
The answer is that both “desu” and “des” are 「です」in hiragana.
And, according to the above-mentioned Hepburn Romanization, 「す」is “su”. Therefore, you see “sushi” or “desu” in Japanese textbooks. Indeed, 「す」for “sushi” pronounced by non-Japanese and 「す」for” desu” are different phonemes, but the textbooks don’t take it into consideration. The writing style which Japanese are familiar with is more important than the phonetic way in which non-Japanese pronounce easily.
Moreover, most Japanese may not realize that 「す」for “sushi” and「す」for” des” have a different sound.
It may be better to know that “Japanese are insensitive to pronunciations”.
In English you pronounce “a” in various ways. For example,
Apple [æpl]
Art [a:rt]
Apartment [əpa:rtmənt]
April [eipril]
But, 「あ」has only one pronunciation. Almost all the hiragana have only one sound (*1), so it is difficult for Japanese to recognize that “a” has several phonemes. Even when we understand this, it is also hard for us to distinguish different phonemes or pronounce them. I think this is one of the reasons why Japanese are not good at pronouncing foreign languages.
I often see Non-Japanese, who are more sensitive to pronunciations than Japanese, write Japanese words with the roman alphabet in their own way. They try to write them as close as possible to what they hear. Even so, I think that is fine when considering pronunciation. In textbooks Japanese phonemes are written in alphabets only for people who don’t know Japanese at all, and they sound similar, but, are not completely the same. Therefore, you need to listen to Japanese carefully and acquire the pronunciation.
But, it is maybe better to write in hiragana or alphabets with Hepburn Romanization when you learn how to conjugate verbs and adjectives.
Alphabetic writings such as Hepburn Romanization are used for our passport names, station names, or street signs. And they are used when we type in Japanese on a computer. We type “ni ho n” for 「にほん（日本）」, “to u kyo u” for 「とうきょう（東京）」.
I row (I, ki, shi, chi…) and U row (u, ku, su, tsu…) : The vowels are sometimes weaker when they are paired with K (ka, ki, ku, ke ko), S, T, H or P.
すき (like): suki → ski
ふかい (deep): fukai → fkai
あさくさ (Asakusa): asakusa → asaksa
The vowels that follow K, S, T, H or P are weaker when they are at the end of the words.
～ます (the end part of a verb in distal style): masu → mas
かく (write): kaku → kak
まつ (wait): matsu → mats
When “u” is placed after O, “u” is pronounced as “o”. Long “o”.
とうきょう (Tokyo): toukyou → tokyo
おとうさん (father): otousan → otosan
When “I” is placed after E, “i” is pronounced as “e”. Long “e”.
えいが (movie): eiga → ega
けいたい (mobile phone) : keitai → ketai