Why is the letter 'Q' almost always followed by the letter 'U'?

Solution 1:

There is no rule that q must be followed by u in all circumstances. This is merely true in the vast majority of circumstances, and it goes back to Latin.

The early Latins had three different letters for the [k] sound: C K Q. However, they only had one letter to represent the [u] and [w] (or [v]) sounds: V. It became customary to write the sequence [kw] (which is fairly common in Latin) as QV and all other instances of [k] as C. (K dropped out of use in most words.) This usage survived into most other European languages that were written with the Latin alphabet, though eventually the letter V was differentiated into U and V, and the accepted spelling of [kw] became QU.

Words spelled with Q without U are generally more recent additions to English, and often represent words borrowed from Semitic languages. Those languages are written with non-Latin alphabets and often have more than one [k]-like sound. When transliterating these scripts, K is usually used for [k], and Q for another sound such as [q], a uvular, "guttural k". In romanizations of Chinese Q is also used for a sound similar to the English "ch".

Solution 2:

In English, there are no rules about when the q must be followed from a u.

English words use qu when it was present in the original word.

English Word | Original word | Language
quandary     | quando        | Latin
quantify     | quantificare  | Medieval Latin
quantum      | quantus       | Latin
quark        | quark         | German
quarry       | quarreria     | Medieval Latin
quest        | queste        | Old French
question     | question      | Old French
query        | quaerere      | Latin
quetzal      | quetzalli     | Aztec

Other times, English words use qu instead of cw, ku of the original word.

English Word | Original word | Language
quean        | cwene         | Old English
quell        | cwellan       | Old English
queen        | cwēn          | Old English 
quench       | -cwencan      | Old English
quern        | cweorn(e)     | Old English
quiche       | Küchen        | Alsatian dialect
quick        | cwic, cwicu   | Old English

Solution 3:

I had a discussion on this topic with my friend who is a linguist and he explained it thoroughly. Here's the summary of what he said:

Why is the letter Q usually followed by the letter U:

The Ancient Greeks got the alphabet from the Phoenicians, who had both /k/ and /q/ as consonants. Greek got these as K (kappa) and Ϙ (qoppa), but Greek only had /k/. So instead, some Greeks used qoppa to represent /k/ before back vowels [u], [o] and [ɔ] probably because they caused the /k/ to sound deeper, closer to /q/.

Then, the Etruscans got the alphabet from these qoppa-using Greeks. The Etruscans didn’t distinguish /k/ and /g/ in their language, so C (the descendant of Greek gamma /g/) got thrown into the mix, too. So, when the Latins inherited this version of the alphabet, they ended up with C, K, and Q, which was rather redundant. In Latin, the original norm was:

for /g/, use C
for /ka/, use KA
for /kw/ and /ku/, use QV
for /ko/, use QO
for any other /k/, use C

Later, the K fell out of fashion, G was invented to represent /g/, and Q remained only for QV /kw/. Everywhere else, /k/ became C (except in rare cases like the word kalendae (first day of the month), because abbreviating it as K was the norm).

On the other hand, Old English words with /kw/ were originally written with cw (or rather, cƿ), like cwēn (queen), but when French brought its spelling influences inherited from Latin to England in the Norman Conquest, even native English words with /kw/ came to be spelled with QU. And of course, any foreign words spelled with QU usually keep it when they enter English.

Words containing the letter Q not followed by U:

In English, the letter Q is usually followed by the letter U, but there are some exceptions. The majority of these are anglicised from Arabic, Chinese, Hebrew, Inuktitut, or other languages that do not use the English alphabet, with Q representing a sound not found in English. For example, in the Chinese pinyin alphabet, qi is pronounced /tʃi/ (similar to "chi" in English) by an English speaker, as pinyin uses "q" to represent the sound [tɕʰ], which is approximated as [tʃ] (ch) in English. In other examples, Q represents [q] in standard Arabic, such as in qat, faqir and Qur'ān. In Arabic, the letter ق, traditionally romanised as Q, is quite distinct from ك, traditionally romanised as K; for example, قلب /qalb/ means "heart" but كلب /kalb/ means "dog". However, alternative spellings are sometimes accepted, which use K (or sometimes C) in place of Q; for example, Koran (Qur'ān) and Cairo (al-Qāhira).

Of the 72 words in this list, 68 are nouns, and most would generally be considered loanwords; the only modern-English words that contain Q not followed by U and are not borrowed from another language are freq, qiana, QWERTY, and tranq. However, all of the loanwords on this list are considered to be naturalised in English according to at least one major dictionary (see References), often because they refer to concepts or societal roles that do not have an accurate equivalent in English. For words to appear here, they must appear in their own entry in a dictionary; words that occur only as part of a longer phrase are not included.

[List of English words containing Q not followed by U - Wikipedia]