moun - tain - ous
There are many reasons to learn how to divide mountainous into syllables. Separating a word like mountainous into syllables is mainly to make it easier to read and pronounce. The syllable is the smallest sound unit in a word, and the separation of the mountainous into syllables allows speakers to better segment and emphasize each sound unit.
Knowing how to separate mountainous into syllables can be especially useful for those learning to read and write, because it helps them understand and pronounce mountainous more accurately. Furthermore, separating mountainous into syllables can also be useful in teaching grammar and spelling, as it allows students to more easily understand and apply the rules of accentuation and syllable division.
In the case of the word mountainous, we find that when separating into syllables the resulting number of syllables is 3. With this in mind, it's much easier to learn how to pronounce mountainous, as we can focus on perfecting the syllabic pronunciation before trying to pronounce mountainous in full or within a sentence. Likewise, this breakdown of mountainous into syllables makes it easier for us to remember how to write it.