Learn Finnish: Lapsi ei pelannut jalkapalloa, koska hän ei lämmitellyt. - The child did not play football, because he did not warm up.

Pelata is a Type 4 verb. To form the active past participle (the word used after ei for the negative past tense), you add -nut or -nyt to the consonant stem. The consonant stem of pelata is pelat-. The t at the end of the stem assimilates (changes to match) the n in the ending, creating a double nn. So, pelat- + -nut becomes pelannut.

How do we get lämmitellyt from the verb lämmitellä (to warm up)?

Lämmitellä is a Type 3 verb, and its consonant stem is lämmitel-. When we add the past participle ending -nyt, the n in the ending assimilates to match the l at the end of the stem. This changes lämmitel- + -nyt into lämmitellyt.

Why does lämmitellyt end with a y (-lyt) while pelannut ends with a u (-nut)?

This is because of vowel harmony. The vowels in pelata include the back vowel a, so it takes the back vowel ending -nut. The vowels in lämmitellä are all front vowels (ä, i, e), so it requires the front vowel ending -nyt. After the consonant assimilation changes the n to an l, it leaves us with -lyt.