Basic Rules:

If a word ends in a vowel, n, or s the spoken stress is on the 2nd to last syllable.

Example: como, hablan, ustedes

If a word ends in any consonant other than n or s the spoken stress is on the last syllable.

Example: universidad, conversar, animal

If a word does not follow these rules a written accent mark is placed over the stressed syllable to show which syllable to stress

Examples: teléfono, México, Gómez, último, dé, veintiséis