1. There is almost no resemblance between sein and it's conjugated forms! We call verbs that don't follow the pattern of conjugation, irregular verbs.
2. The good news is, sein is so frequent that you will learn it very quickly.
3. Second good news is, there is no other verb as irregular as sein.
The rest of the verbs will be irregular just in the forms of du and er/sie/es, the others will always be super regular and predictable! So it is still worth it to learn those endings!