Words ending in a vowel, ‘n’ or ‘s’ are stressed on the penultimate syllable.

Words ending in a consonant (not ending in ‘n’ or ‘s’) are stressed on the last syllable.

Words that break these two rules have an accent

For example, ‘I ate’ in Spanish is ‘comí’. You need the accent on the ‘i’ as the stress is on the last syllable and ‘comí’ ends in a vowel; it therefore breaks the first rule, and needs an accent.   

Similarly, the word ‘estación’ (station) needs an accent on the ‘o’ as the stress is on the last syllable and ‘estación’ ends in an ‘n’; it also breaks the first rule, and needs an accent.

The words ‘lápiz’ and ‘árbol’ both need accents as they both end in a consonant that isn’t ‘n’ or ‘s’, and both aren’t stressed on the last syllable; they therefore break the second rule, and need an accent.