"last tree"

Translation: poslední strom

[JanisaChatte]

Why is it poslední and not posledný here?

[Aldrost]

According to this: http://www.locallingo.com/czech/grammar/adjectives.html
(and the Tips & Notes in Masculine, too)

Czech adjectives are divided into two groups:

1) Adjectives with a hard ending in the nominative singular
-ý (masculine)
-á (feminine)
-é (neuter)

2) Adjectives with a soft ending in the nominative singular
-í (masculine, feminine and neuter)

All Czech adjectives end with a long vowel.

It seems n is considered a soft sound.

[Aldrost]

Though I just came across another example where this does not hold: https://www.duolingo.com/comment/25965954

jiný muž

[AgnusOinas]

/n/ is a hard sound, but /ň/ is a soft sound. "Poslední" has the palatal /ň/ sound in it, but in spelling this is shown by using "i/í" (instead of "y/ý"), it's not shown on the letter "n" itself.