Ist vs Isst
"Ist" can be the third person singular part of the verb "sein" (to be) which has not been covered yet. "Isst" or "Ißt" is the third person singular part of the verb "essen" (to eat). What difference is there in pronunciation? A teacher told me once that where there is a double s, the sound is longer. Is that correct?
Pronounciation is exactly the same, both with a short "i" [ɪ]. There's a pun/proverb on this:
"Man ist was man isst" (You are what you eat).
In generally doubled consonants such as "ss" force a sharp pronounciation of the consonant (here [s] as opposed to [z]) and a short pronounciation of the preceding vowel (here [ɪ] as opposed to [i:]).
The vowel preceding "st" is also normally pronounced short (hence the same pronounciation of "ist" and "isst") and "st" is normally pronounced [st] or [ʃt] at the beginnning of a word.
It can sound a little bit longer, but it really will depend on the context of the sentence. Pronounciation is exactly the same :)
In 1981, when I first learned German, what you said is right.
But between 1996 and 2005, many rules regarding "ss" were revised, and currently, "isst" is the correct spelling and, yes, "isst" rhymes with "ist."
And in a spoken sentence, if the predicate noun is female or other, there will be no show of accusative case to indicate the context.