Do the "sei" sound and the "se" sound, sound the same?

koopashell-san, konban wa.

In theory, the sei sound is two syllables long, and has an extended "e" sound. In practice:

(1) The "ei" sound often lasts less than twice as long the "e" sound-- but at least one and a half times as long

(2) It is sometimes a pure "ee" sound, and sometimes has a glide into an "i" sound, as the spelling in roomaji would suggest. This depends both on the word it occurs in, and on the particular speaker.

I would advise pronouncing it as twice the length of an "e" sound, and without the "i" glide-- this is a good way to make sure it is not mistaken for a short "e," but so that it doesn't sound like the English long "a" (as in the word "late" ).

--Tony

sensei - teacher

As for the latter part of the word "sei", you pronounce it "se-i" like "sa" from English word "same" if you do slowly. But when the word is pronounced faster, your ears might catch as "se". Namely, the word "sensei" sounds like "senseh". If so, your ears work properly, I think.

Shizu

Thank you Mr. Tony and Shizu,

but what about -ou?

Same thing. In theory, it is just a longer "o" sound-- two syllables worth. In practice, it is sometimes that, and sometimes an "o" sound gliding into a "u" sound, like the long "o" sound in English. And as Shizu says, if it is pronounced quickly, it is hard (at least for us English speakers) to hear the difference between it and a single "o."

By the way, here is an (maybe) exception:
If it is a verb which ends in "ou", the "u" is pronounced more clearly.

Example: "ou"
If the word is a noun which means "king", you pronounce it like "oo" unless you do it so slowly. On the other hand, if it is a verb which means "chase", the word is pronounced like "ou".

Some examples:
niou - smell
sasou - invite
oou - cover

Perhaps it doesn't matter even if the verb's last "u" like a long vowel of "o", but it might invite misunderstanding at times. For example like a sample "oou": if it's pronounced like "ooo", someone may regards it as an imperative form of another verb like "Let's chase!", or different expression like "oh!" in English or something like that depending on the situation, lol.

Shizu