I think the others explained the concept of a regular or irregular verb pretty well. As @tienhung1985 said, you can’t tell just by looking at the verb; you just have to memorise them. I really like the website conjuga-me.net to learn and look up verb conjugations. They provide the regular conjugations in black font and the irregular conjugations in orange font. Some verbs like “ser” have irregular conjugations for most tenses but do actually follow the standard rules for some tenses. This website is a quick and easy way to find that information.
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