Theory

The vowel and consonant sounds in Spanish are very different from English.
This means many English sounds are difficult for Spanish speakers to pronounce and this is what generates a Spanish accent and can lead to misunderstandings.
Some examples of these pronunciation difficulties are given below:

S AND ES:
Many English words begin with an S but in Spanish the natural sound is ES.
For example: SCHOOL in English is Escolar in Spanish.
Some Spanish people say ES when they should only say S, especially at the beginning of a word.
SNAKE is not ESNAKE.

I and E:
I in many English words is a short I sound as in SIT.
The short I sound does not exist in Spanish and is replaced by the long E sound.
For this reason words like SHIP and SHEEP are pronounced the same by Spanish speakers.

T and D
This is further complicated by the mixing of the T and D sounds, which means words like CITY and SEEDY sound the same for Spanish speakers.

D and TH:
The sound of D is often confused with the sound of TH, so DAY and THEY sound the same in a Spanish accent.

The last consonant in a word:
Often in Spanish the last consonant in a word is not pronounced.
This can then be done in English where the word ENGLISH can be pronounced as ENGLIS and WANT can sound like WAN.
This can sometimes affect consonants before the last, so KIDS can sound like KIS in Spain.

G and K:
Sometimes the sound of G and K can be confused meaning THINK and THING are often confused or sound the same, and DOG is often pronounced DOK.

SH and S:
SH in English gives a different sound to S but in Spanish the SH sound does not exist.
So S and SH are often confused giving CHIP as SHIP or vice versa.

X and S:
In spanish, X and S sound very similar.
This means that English words like MIX sounds like MIS when pronounced by a Spanish speaker.

Silent H at the beginning of a word:
In Spanish The H at the beginning of a word is not pronounced, so Spanish speakers pronounce HOTEL as OTEL.