Spanish is a very consistent language, but it evolved over time just like English, meaning that there are....irregularities.
*class groans*
They are easy to figure out, though. The trick is to learn the conjugations for all of them apart from the normal Spanish quickies I'll do. The percentages of irregular verbs according to Fluentu state that 5% of -ar verbs are irregular, 72% of -er verbs are irregular, and 33% of -ir verbs are irregular. However, a lot of these verbs are uncommon, and you memorize them by seeing them often in writing. There are several that you should consciously study as they are fairly common and pop up a lot in the tenses:
1.) Ser - to be (I will be talking about the conjugations for this each tense)
2.) Estar - to be (see my note on "ser" - this verb will be discussed later)
3.) Tener - to have, to own
4.) Poder - to be able to (in a sentence, this would be "I can")
5.) Hacer - to do, to make
6.) Ir - to go (this will be discussed later)
7.) Poner - to place, to put
8.) Decir - to say
9.) Ver - to see, to watch
10.) Querer - to want, to love
11.) Saber - to know, to taste/have taste
12.) Llegar - to arrive
13.) Encontrar - to find
14.) Venir - to come
15.) Pensar - to think, to think of
16.) Conocer - to get to know
17.) Sentir - to feel
18.) Recordar - to remember, to recall
These verbs are conjugated irregularly.