Is the past tense for "sink," sank or sunk?
I disagree with the previous answer. They are separate tenses used to express different meanings.
Today I sink the ship. Present tense
Yesterday I sank the ship. Simple past tense
At various times in the past, I have sunk many ships. Compound past (not specific as to when these sinkings supposedly happened)
"Sank" is the simple past - something that happened at one specific past moment in time, sometimes called the Preterit.
"Sunk" is the past participle. It's used with the auxiliary verb "to have".
This verb is from the oldest period of English language development. It's an "inflected" verb, which means it changes its internal spelling as it goes through the tenses.
Past tenses and past participles of verbs from a more modern period of English end in "-ed", such as "worked", "talked" and others: "work, worked, worked", e.g.,
I currently work for a private company.
Last year, I worked for the government.
I have not ever worked for a non-profit.
"Sing, sang, sung" and drink, "drank, drunk" are other examples of inflected verbs from the Old English period.
Today I sing a song.
Yesterday I sang two songs.
By the time the week is over, I "will have sung" many songs.
"Will have sung" is the Future Perfect tense. "Sung" again is the past participle that is used with the auxiliary verb "to have" in the future tense.
Today, I drink.
Yesterday I drank.
I think I "have drunk" too much. (Non-specific compound past tense sometimes called the Imperfect.)
Barron's publishes a series of paperback books for just about every language called "501 Verbs". Probably there's a "201 Verbs", too. I'm sure there's one for English. It will have all the tenses spelled out for the most important verbs.