I'm afraid there is no specific pattern to know when a verb is semi-regular or irregular. They need memorising.
We have lessons about common verbs that are semi-regular or irregular. If you go to your dashboard and type on search at the top right "semi-regular" you will find lots of different lessons about these verbs. I suggest you add them to your notebook, read the lessons and get tested on those.
Vicki - Yes, you are right.
Mohamad - I personally find it helpful that all verbs ending in "-erir" adopt that same vowel-change in the present tense - so, having learned that 'preferir' follows the e>ie pattern, you know that 'inferir', 'conferir', sugerir', diferir, and 'referir' do as well.
It is the same with all the "-vertir" verbs, eg. advertir, convertir, invertir, divertir.
http://dlmcn.com/ir_verbs.html mentions some other verbs: for example those which end in "-egir" or "-edir" are in a different category because they all replace the 'e' with 'i' when conjugating their present tense [of 'yo', 'tu', 'el' and 'ellos'].
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