Students who are new to the English language often struggle with letters that are pronounced differently depending on how they're used in a word. These sounds are called allophones.
In order to understand allophones and how they function, it helps to have a basic understanding of linguistics, the study of language, and phonology (or how sound functions within a language). One of the basic building blocks of language is phonemes. They are the smallest sound units capable of conveying a distinct meaning, such as the s in "sing" and the r of "ring."
Allophones are a kind of phoneme that changes its sound based on how a word is spelled. Think of the letter t and what kind of sound it makes in the word "tar" compared with "stuff." It's pronounced with a more forceful, clipped sound in the first example than it is in the second. Linguists use special punctuation to designate phonemes. The sound of an l, for instance, is written as "/l/."
Substituting one allophone for another allophone of the same phoneme doesn't lead to a different word, just a different pronunciation of the same word. For this reason, allophones are said to be noncontrastive. For example, consider the tomato. Some people pronounce this word "toe-MAY-toe," while others pronounce it "toe-MAH-toe." The definition of "tomato" doesn't change, regardless of whether it's pronounced with a hard a or a softer tone.
Allophones Versus Phonemes
You can distinguish between allophones and phonemes by looking at the letter and how it's being used. The letter p is pronounced the same way in "pit" and "keep," making it an allophone. But p makes a different sound than s in "sip" and "seep." In this instance, each consonant has its own consistent allophone, but they each produce different sounds, making them unique phonemes.
Confused? Don't be. Even linguists say this is pretty tricky stuff because it all comes down to how people pronounce words, not how they're spelled. In other words, you need to pay attention. Paul Skandera and Peter Burleigh, authors of "A Manual of English Phonetics and Phonology," put it this way:
The choice of one allophone rather than another may depend on such factors as communicative situation, language variety, and social class… When we consider the wide range of possible realizations of any given phoneme (even by a single speaker), it becomes clear that we owe the vast majority of allophones in free variation to idiolects or simply to chance, and that the number of such allophones is virtually infinite.
For non-native English speakers, allophones and phonemes prove a special challenge. A letter that has one pronunciation in their native language may sound utterly different in English. For example, the letters b and v have distinct phonemes in English, which is to say they sound different when pronounced. However, in Spanish those same two consonants are pronounced similarly, making them allophones in that language.
"Allophone." British Council, Teaching English.
Burleigh, Peter. "A Manual of English Phonetics and Phonology: Twelve Lessons with an Integrated Course in Phonetic Transcription." Paul Skandera, durchgesehene edition, Print Replica, Kindle Edition, Narr Francke Attempto Verlag; 3, January 18, 2016.
Hughes, Derek. "Phonology: Definition, Rules & Examples." Study.com, 2003-2019.
Mannell, Robert. "Phoneme and allophone." Macquarie University, 2008.