2 letters, 1 sound: Digraphs

Two letters (in written form) can represent a single sound. These are known as digraphs. You probably know them as ch, sh, th, and the like.

English doesn't have a single letter to represent the initial sound ("ch") of the word child, so it must use two letters. Recognizing a digraph is one of the many difficulties in learning a language. For example, take the "ch" sound as mentioned above. One CANNOT simply read it as "a 'c' sound followed by an 'h' sound" - it is a completely different sound altogether.

Here's how the some of the digraphs are represented in phonetic transcription (e.g. using IPA):
Most of the time, only one symbol is used because it represents just a single sound, even though it takes two letters in the written form.

The "sh" sound (as in shell) is represented by [ʃ].
The "ch" sound (as in church) is represented by [tʃ]. Two symbols are used but it is still a single sound. It is actually a combination of the "t" and "sh" sounds. Try making a "t" sound and a "sh" sound simultaneously and the result will be a "ch" sound.

In some dictionaries and texts, [š] and [č], are used to denote the "sh" and "ch" sounds, respectively.

There are many more digraphs, and they also exist in languages other than English.

Consonants and Vowels

What's the difference between a consonant and a vowel?

In grade school, I'm pretty sure you've been taught that there are 5 vowels: AEIOU. The rest are consonants. However, this distinction fails to take into account the sounds of the letters. There are more sounds in English than there are letters (in the written form). The sounds and letters of a particular language do not always form a 1 to 1 correspondence, and this is why we must use 2 letters to represent a single sound (such as "sh").

How we produce speech sounds

The process in which we produce sound by using our vocal tract is much like that of a wind instrument. Each sound differs from another sound by a unique combination of features: the way you shape your mouth and tongue and move parts of the vocal apparatus when you make the sound. Air coming from the lungs passes through the vocal tract, which shapes it into different sounds. Then the air exits the vocal tract through the mouth and/or nose.

What differentiates a consonant and a vowel in terms of speech sounds?

Quite simply: Consonants are pronounced by obstructing the airflow through the vocal tract.

(Note: the details of how to pronounce every sound is beyond the scope of this site)

Example:
The [p] sound is made by obstructing air at the lips. When you say the word put, air is built up behind the lips and then released.

On the other hand, vowels are different from consonant sounds in that they are produced by passing air through different shapes of the mouth and different positions of the tongue and lips.

Examples:
For the "ahhh" sound (the vowel in the word pot, IPA [ɑ]) the tongue's position is in the lower back of the mouth.
For the "eeeee" sound (the vowel in the word be, IPA [i]) the tongue's position is in the upper front of the mouth. (This is why we say cheese when we take pictures!)

How negative prefixes are determined in English

Ever wonder why certain words use im- and others use in- for their negative prefixes?

This topic requires a bit of knowledge on phonetics and phonology, which was initially discussed in this post about phonetics.


im-
Example words: impractical, imbalance, improper, impressive

Notice how words that take the im- negative prefix start with p and b. The p and b sounds are made with the lips, and this is the same place of articulation as the m sound. This is known as assimilation: the place of articulation of the final sound of the prefix ("m") matches the place of articulation of the initial sound of the root word.

in-
Example words: inaccessible, involuntary, insincere, infamous

In most other cases, the in- negative prefix is used. This is because n is an alveolar consonant, pronounced with the tongue against the area just above the upper teeth. Unlike the p and b sounds, this is a central area, and thus the in- prefix can "handle" many other sounds.

il-, ir-
Example words: illogical, illegal, irrational, irresponsible

The l and r sounds are known as liquids. Just like the im- prefix, assimilation happens here. In this case, the l or r is simply doubled up in forming the prefix.