Vowel-final and consonant-final present stems
Use the present stem, not the infinitive’s appearance alone, to understand -ю/-у and -ют/-ут.
A vowel-final stem such as чита- gives читаю. A consonant-final stem can take -у or -ю according to its spelling and softness. Learn the verified present stem instead of assuming that every verb is made by removing -ть.
- The visible infinitive is only a starting clue.
- The present stem is the useful form-building base.
- Softness can also affect whether ю appears.
Why колоть becomes колю
The person ending is attached to the verified present stem, not to the pronoun я.
колоть → present stem кол-1st-person singular ending: -юкол- + -ю → колюMini-check
Which base should guide a finite present form?
More detail: stem softness
A technical analysis can mark the final consonant as soft. Beginner notation does not need special phonological symbols: the useful written build is кол- + -ю → колю.