Learn Turkish: Dalgalar tekneyi salladı, hemen kıyıya dönmek gerekiyordu. - The waves shook the boat; it was necessary to return to the shore immediately.

To say "it was necessary to do something" in the past, you use the infinitive form of the verb ending in -mak or -mek (here, dönmek) followed by gerekiyordu or lazımdı. Literally, the structure translates to "to return was necessary."

Could I use lazımdı instead of gerekiyordu in this sentence?

Yes, absolutely! Saying kıyıya dönmek lazımdı means exactly the same thing. Both words are used with the infinitive to show a general necessity in the past.

Dalgalar (waves) is plural, so why isn't the verb pluralized as salladılar?

In Turkish, when a plural subject is non-human (like animals, objects, or nature like dalgalar), the verb usually stays in the singular form (salladı). The plural verb endings (-lar / -ler) are generally only used for human subjects.

Why is there a -yi at the end of tekneyi?

Tekne (boat) is the direct object receiving the action of the shake. Because it is a specific boat ("the boat"), it needs the definite accusative case. We add the buffer -y- followed by the vowel -i, making it tekneyi.