The letter h may be the easiest of all the Spanish letters to pronounce: With the only exceptions being a very few works of obvious foreign origin, the h is always silent.
Note, however, that the letter combinations ch, which used to be considered a separate letter of the alphabit, and the sh in flash and possibly a few other imported words, are pronounced basically the same as in English.
But the usual silence of h doesn't mean its pronunciation doesn't sometimes trip up beginning Spanish students. Those who speak English as a first language often want to pronounce the letter when it is in a cognate, that is, a Spanish word that is more or less the same as English. For example, the h should not be pronounced in words such as vehículo (vehicle), Habana (Havana), Honduras and prohibir (prohibit), as tempting as it may be.
This is further explained in our brief audio lesson on the h. Phrases used by native speakers in the audio are "hasta mañana" and "no hay de que."
If the h is silent, why does it exist? For reasons of etymology (word history) only. Just as the "k" in the English "know" and the "b" in "lamb" used to be audible, the Spanish h used to be pronounced ages ago. Almost all Spanish consonants have become softer over the years; the h became so soft as to become inaudible.
The Spanish h also was used to separate two vowels that weren't pronounced as one, that is as a diphthong. For example, the word for "owl" used to be spelled as buho to indicate that it was pronounced as two syllables rather than rhyming with the first syllable of cuota or "quota." Nowadays, though, an accent is used over a stressed vowel to indicate the lack of a diphthong, so the word is written as búho. In this case, then, the accent isn't used to indicate stress as it usually does, but as a guide to proper pronunciation of the vowels.
Also, these days it is quite common for the h between vowels to be ignored in pronunciation; that is, the vowels sometimes run together despite the h between them, depending on how they are stressed. For example, prohibir is pronounced more or less the same as proibir would be. Note, though, that when the stress is on the second syllable in forms of this word, it is accented and pronounced clearly. Thus conjugated forms of the verb include prohíbes, prohíbe and prohíben.
Exceptions: The words where the h is pronounced? Apparently the only such word that is recognized by the Royal Spanish Academy as fully Spanish is hámster, a cognate of the English word for "hamster," although it came to Spanish by way of German. It is pronounced much as it is in German or English, as if it were spelled jámster.
Other imported words, listed by the Academy as foreign or not listed at all, in which natives speakers often pronounce the h include hockey (not to be confused with jockey), hobby (plural usually hobbys), Hong Kong (and some other place names), hacker and hit (baseball term or a major success).
Also, jalar and halar (to pull) are often used synonymously, and in some regions it is common to pronounce jalar even while writing halar.