The four imperfect-verb prefix letters are أ، ن، ي، ت.
أَكْتُبُ — I write.
نَكْتُبُ — We write.
يَكْتُبُ — He writes.
تَكْتُبُ — You write, or she writes.
These illustrate the prefixes only. A noun beginning with one of these letters is not automatically diptote.