Learn Finnish: Asiakas ei ilmoittanut hammaslääkärille, koska hän ei muistanut. - The customer did not inform the dentist because he did not remember.

In Finnish, the negative past tense is made of two parts. First, the negative verb matching the person (here, ei for he/she). Second, the past participle of the main verb, which usually ends in -nut or -nyt. You do not use the past tense marker -i- like you do in positive sentences.

Why does ilmoittanut have a double T, if the 'I inform' form (ilmoitan) only has one?

This happens because of consonant gradation. For Type 1 verbs, the past participle ending -nut attaches to the strong stem. So, you use the strong grade (with tt) for ilmoittanut, even though the present tense ilmoitan uses the weak grade (with one t).

Why does the word for dentist end in -lle (hammaslääkärille)?

The verb ilmoittaa (to inform) indicates who is receiving the information by using the allative case, which adds the ending -lle (meaning 'to' or 'onto'). You are literally giving information 'to the tooth doctor' (hammas is tooth, lääkäri is doctor).

What would the verbs look like if this sentence were positive?

They would be ilmoitti and muisti. In the positive past tense, you use the past marker -i- attached to the verb stem (like Asiakas ilmoitti). The -nut ending seen here is only used when the past tense is negative (or in perfect tenses).