Learn Portuguese (Portugal): É difícil comer só um bombonzinho depois do pudinzinho. - It is difficult to eat only one little bonbon after the little flan.

The original words are bombom and pudim. In Portuguese, the letter m is only written at the very end of a word or before a b or p. When we add the suffix -zinho, the nasal sound is no longer at the end of the word, and the following letter is z. To follow the spelling rule, the m must change to an n.

Does the pronunciation of the nasal vowel change when the m becomes an n?

No, the nasal sound remains exactly the same. The -on in bombonzinho sounds just like the -om in bombom, and the -in in pudinzinho sounds like the -im in pudim. The spelling change is just a rule of written Portuguese.

Why is it depois do and not just depois o?

The word depois requires the preposition de when it is followed by a noun, forming the phrase depois de (after). Because we are saying after the little flan, we combine de with the masculine word for the, o, which merges into do.