A syllable is the smallest piece of a word which contains the sound of a vowel, so BANANA has three syllables:
BA NA NA.
When English is spoken correctly, one and only one syllable is always stressed in every word, and stressed means it is pronounced longer and slightly louder, and its vowel sound is clearer.
The word NATURE has two syllables: NA and TURE, and the syllable NA is stressed, so the word is pronounced:
When a syllable is stressed in this way, the other syllables become weak and their vowel sounds very often become neutral, and sound like SCHWA.
In the word NATION,
NA is stressed and the A sound is clear and strong.
TION is not stressed and sounds like shun (u = schwa)
So, NATION is pronounced NA shun.
If a mistake is made in which syllable to stress, the word becomes very difficult to understand, so as you learn vocabulary, not only should you learn the meaning of a word, but also which syllable is stressed, so you can pronounce the word correctly.
As prefixes and suffixes are added to words, the stressed syllable sometimes changes, and a SECONDARY STRESS is used:
INTERNATIONAL has five syllables, IN TER NA TIO AL
and the main stress is on the third syllable NA. The second, fourth and fifth syllables all have the schwa vowel sounds, but the first syllable has a secondary stress and sounds like IN.
so INTERNATIONAL = IN tu NA shun ul
There is a rule in English that doesn't allow words to start with two unstressed syllables, so the first is given a secondary stress.
Sometimes using stress and SCHWA makes some syllables disappear and others appear:
the word COMFORTABLE should have four syllables:
COM FOR TA BLE, but when you say the word correctly, it's pronounced
COMF tu bul, and the second syllable in COMFORTABLE disappears.
The word ATHLETE should have two syllables:
ATH LETE, but the word should be pronounced
A thu LETE, with a secondary stress on the A and a false syllable before LETE.
This can sometimes make its spelling confusing.