It's an irregular verb, meaning it doesn't follow the standard rules of verb conjugation. Here's how it works:
* Present tense: I sink, you sink, he/she/it sinks, we sink, you sink, they sink
* Past tense: I sank, you sank, he/she/it sank, we sank, you sank, they sank
* Past participle: sunk or sunken
Let me know if you have any other questions about verbs!