The kids I used to teach had trouble producing the sound of short “i”. It comes out as long “ee”. (In linguistics, this ee and i are a tense/lax vowel pair.) Thus, as I tell new teachers during training, there are no fish in Singapore. They’re all feesh.
That means that “ship” and “sheep” are homophones. The fact that “ship” and “sheep” are not actually the same word is really confusing to kids who are learning plurals and collective nouns (fleet of ships, flock of sheep).
Another significant effect of this problem is that “his” and “he’s” sound exactly the same. The obvious effect of this confusion is that kids often write one of these words when they should be writing the other one. The more subtle effect of this confusion is that kids sometimes assume that there exists a possessive adjective “she’s” which means “her”.
Here’s what they hear here:
He is a boy. That bag is he’s bag.
Therefore, by analogy, they want to say:
She is a girl. That bag is she’s bag.
I wish English were that logical!
I think (I hope?) most Singapore kids grow out of saying “she’s” as a possessive adjective but they don’t necessarily learn to pronounce lax vowels as lax vowels. The adults here also say “feesh”.
The “oo” in “moon” and the “oo” in “book” are another tense/lax pair, which explains why kids (and adults) say the word “book” with the vowel sound that’s in “moon”.