1. Diphthongs
A diphthong is a glide or blend from one vowel position to another when two vowels come beside each other in a word. Of the five Spanish vowels,
a, e, and o --- strong
i and u --- weak
2 strong beside each other-- two separate sounds real, Paos
2 weak beside each other-- diphthong (blend) triunfo, ciudad, veintiuno, ruina, Luis
1 weak and 1 strong-- diphthong (blend) seis, auto, Eduardo, Miguel, radio, gracias

2. Stressed & Accented Words
Most words ending in a vowel, -n, or
–s are stressed on the next-to- last syllable.
(cla-ses, co-mo, re-pi-tan)
Most words ending in a consonant other than –n or –s are stressed on the last syllable.
(fa-vor, se-ñor, us-ted, es-pa-ñol)

3. (ca-fé, a-quí, nú-me-ro, in-glés)
Words that are stressed in any other way have a written accent mark on the vowel of the syllable that is stressed.
(ca-fé, a-quí, nú-me-ro, in-glés)
Written accent marks are also used to mark the difference between pairs of words spelled the same.
(el-the/él-he, si-if/sí-yes, tu-your/tú-you)