Which suffix requires the use of combining vowel?

The suffix that requires the use of a combining vowel is -al.

Here's why:

* Combining vowels are used to connect a root word with a suffix that starts with a consonant. They make the pronunciation smoother.

* The suffix "-al" starts with a consonant ("l").

* Therefore, when adding "-al" to a root word ending in a consonant, a combining vowel (usually "i" or "e") is needed to make the word pronounceable.

Examples:

* nation + al = national

* logic + al = logical

Let me know if you'd like more examples or want to explore other suffixes!