I've just returned from a trip to the Galapagos Islands. There were many Hispanics speaking English, and in general they were easy to understand, but I observed some aspects of their pronunciation which produce a particular accent. The accent of a non-native speaker is the result of a combination of many factors, but I think that a review of a few of them is very important to acquire more effective communication in English.

The points which I'm going to submit for consideration touch upon three difficulties which necessarily present themselves to a Spanish speaker who intends to achieve a satisfactory pronunciation of English. The two first points have to do with the production of characteristic sounds of the English language which don't exist in Spanish. The third difficulty is of a distinct nature, as we will see below.
 

First difficulty: voiced "s"

The sound of a "z" in English is like that of an "s" in Spanish, but voiced. "Voiced" means that a sound contains a tone produced by vibration of the vocal chords. Here's a good video in YouTube which shows the production of this sound:

The sound of the English "z" does not exist in Spanish, so that Spanish speakers tend to pronounce it like an "s". One can perceive the difference between these sounds in the English words sip and zip (I'll indicate English words used as examples with italics).

To make matters worse, the sound of a voiced "s" is often represented in English by the letter "s"! Perhaps the most common words in English with a voiced "s" are conjugations of the verb to be in the third person singular, in the present he is, as well as in the past he was. In the words is and was, the correct pronunciation is "iz" and "waz", and not "iss" and "wass", as Spanish speakers are in the habit of saying. (I'm indicating the pronunciation with a "z"; naturally the spelling is still with an "s".)

Another case in which a voiced "s" is used is in the formation of the English plural by the addition of an "s". When the singular word ends with a voiced sound, the "s" of the plural is also voiced. Thus, in bird - birds, job - jobs, and bag - bags, etc, the "s" is voiced (the pronunciation is like "birdz", "jobz", and "bagz"), but in the English word rat, the letter "t" isn't voiced, so in the plural rats, the "s" isn't voiced (hence the pronunciation is "rats").

The same rule applies for the "s" at the end of the third-person singular of English verbs. If the sound prior to the "s" is voiced, the "s" is voiced as well: she sings ("singz") and she loves ("lovz"), but he eats ("eats").

When a plural is formed by adding an "es" (to words which end in "s", "x", "ch" or "sh"), the "s" is always voiced (kiss,"kissez"; box, "boxez"; church, "churchez"; dish, "dishez").

An initial "s" in English is never voiced, nor is a double "s" (ss).

In general, it's important to consult a dictionary to verify the pronunciation of an English word. Here are some common English words with a voiced "s":

Second difficulty: the sound of the letter "i"

There are not many English words in which the letter "i" is pronounced like the letter "i" in Spanish. There is the word ski, and words which end in -ing (including gerunds, or words like ring, sing, and wing), but the majority of the letters "i" in English are pronounced in two different ways:

A long "i" in English is pronounced like "ai" in Spanish. This isn't difficult for a Spanish speaker. Some words that contain a long "i" are: I, fine, mine, and tile.

However, the sound of the short "i" is a sound that doesn't exist in Spanish. This sound appears in the important English words if, in, is, and it. Here's a good video in YouTube which demonstrates this sound:

The sound of an "i" in Spanish is not the same. If you pronounce the word it with the sound of the letter "i" in Spanish, the pronoun it is changed to the verb eat, which has a completely different meaning. You can practice the difference between the two sounds with the imperative sentence Eat it!

It's important to remember that the vast majority of words which use the letter "i" in English don't use the sound of the Spanish "i". Consult a dictionary!

In general, an initial "i" or an "i" between two consonants is a short "i", but the addition of a final "e" changes the pronunciation into a long "i":

But in the word "give" the rule is not observed - the "i" is short. There are words like live with two pronunciations, with a short "i" or a long "i", depending on the meaning. English would not be the language we know without multiple exceptions.

Third difficulty: impure "s"

The third difficulty of pronunciation in English comes from English words which start with an "impure s", which means an initial "s" followed by a consonant. Some examples are student, school, space, and ski. The corresponding Spanish cognates, in general, don't start with an "s", but rather with an "es": estudiante, escuela, espacio, and esquí. There is thus a tendency for Spanish speakers to pronounce these English words with the sound of an "e" before the "s", as if the words were: estudent, eschool, espace, and eski. This is not correct.

Here are some good discussions of this issue:

In a Zodiak boat in the Galapagos Islands, I heard an Ecuadorian naturalist identify a bird in English with the name Striated Heron. The naturalist said "estriated", but an American passenger didn't understand what she had said. So the naturalist repeated the first word, and I was surprised to hear that she stressed the initial "s" by saying Estriated, prefixing the sound of the letter "e" even more strongly than the first time. The passenger was even more confused.

In English, there is no word that starts with the sound of an "e" when the spelling of the word starts with an impure "s". To emphasize that an English word begins with an "s", it's better to think SStriated! Practice English words which start with an impure "s" by beginning with the hiss of "s" alone at the start of the word. The proper pronunciation is thus SStudent, SSchool, SSpace, and SSki. There is no "e" sound at the beginning.

Conclusion

The English language uses more sounds than Spanish. The pronunciation of Spanish is more logical, with only one sound for each letter. Certainly, there are various factors which produce a Spanish accent in English, but I think I've identified three of the most important difficulties. I hope that this page helps my Spanish speaking readers to obtain a better pronunciation of English.