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.
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.
Your first sentence is "The woman IS an egg" and your second sentence is "The woman EATS an egg."
That is the topic of this blog, that the current spellings and pronunciations of "ist" and "isst" are too similar.
Thirty, almost forty years ago, when I lived in Germany (near Kaiserslautern), "isst" was spelled "ißt" and pronounced more like "east," but no more.
Context may help since "ist" will always take nominative case, while "isst," when it takes an object, will have an accusative object. However, only masculine nouns show accusative case.
Which is why, in your examples, case never showed up to tip you off as to what was going on in the sentences.