Interactive pronunciation book
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Same shape of problem as the last unit. Books, dogs, buses. One spelling, three pronunciations, and one of them adds a syllable.
This ending appears on plurals, on third-person verbs and on possessives, so you produce it constantly. And because -s often lands in a consonant group, it is a common place to lose a grammatical ending entirely.
If your language does not allow final consonant groups, you will either add a vowel, books becoming bookis, or drop the s, losing the plural.
If your language has s but not z at the end of words, you will use s everywhere. This is less serious than the other two, because it rarely confuses anyone, but it is audible.
Spanish speakers in particular may weaken or drop final s, since many varieties do this natively.
The bigger risk is not substituting but dropping the s and losing the plural. Spanish speakers especially weaken a final s.
Exactly the same logic as the last unit, and saying so out loud helps.
If the word ends in an s-like sound, s, z, sh, ch, j or x, add a syllable. Buses, watches, changes, prices.
If the word ends in any other voiceless sound, p, t, k, f or th, the ending is an s. Books, cats, stops, months.
If the word ends in any other voiced sound, the ending is a z. Dogs, plays, rivers, needs.
Throat test again. Buzz before the ending means z. And note that the z ending is the most common one by a wide margin, because most English sounds are voiced. If you are using s everywhere, you are wrong most of the time.
Then check for dropped endings specifically. Record a sentence and confirm every plural is actually there. That is where the ending most often disappears.
Throat test again: a buzz before the ending means z. Then record a sentence and confirm every plural is actually there.

Pick anything above, then record yourself saying it. Your recording appears in red over the model in gray. You are comparing shapes, not chasing a score.
Units 26 and 27 have the same rule. What is it?