Khmer

Khmer (Cambodian) is the official language of Cambodia. It forms its own Khmeric branch in the Austroasiatic language family, making it (somewhat distantly) related to Vietnamese, Mon, Khmu, Wa, Palaung, Khasi, Santali, Mundari, and other languages.

Within Cambodia, Central Khmer is the basis for the standard language. Northern Khmer is spoken north of the border in the southern part of Thailand's Isan region. Southern Khmer is spoken south of the border in Vietnam's Mekong Delta. Western Khmer is spoken in the Cardamom Mountains in western Cambodia. The dialect of Phnom Penh, the capital of Cambodia, also has a number of unique colloquial features that distinguish it from the standard language.

Khmer is written in the Khmer script, which is ultimately derived from the Indic Brahmi script through the southern branch. As such, it is useful for learners to learn some fundamental principles underlying the typical Indic script system.

Consonants

Consonants in the Khmer script are organized into two series, the Ą Series and the Ǫ Series. The name refers to the vowel that the consonant takes as its inherent vowel, with Ą Series consonants taking /ɑː/ ( ~ /ɒː/) by default Ǫ Series consonants taking /ɔː/ by default.

Unlike Sanskrit and Pali, Khmer does not make a phonetic distinction between retroflex and dental consonants.

The two glottalized implosives /ɗ/ and /ɓ/ can vary with normal voice plosives /d/ and /b/.

វ can vary between /ʋ/ and /w/.

ឞ* and ស* are rare, used for transcribing Indic words.

The apostrophe used in Lexx Rom for glottal initial អ can be omitted at the beginning of the word, but should be used if within a word.

From the above, one can see that generally speaking, ancestrally voiceless initials induced Ą Series vowels, and ancestrally voiceless initials induced Ǫ Series vowels. However, there are exceptions, as Khmer reutilized some duplicate letters to serve as the alternate vowel series:
The unaspirated retroflex plosives ដ and ឌ are repurposed for /ɗ/ Ą Series and Ǫ Series.
The retroflex nasal ណ is repurposed for /n/ Ą Series to complement ន Ǫ Series.
The retroflex lateral ឡ is repurposed for /l/ Ą Series to complement ល Ǫ Series.
ប is used as a base letter for /ɓ/ in the Ą Series.

Even with the repurposing of these letters, there still remains some gaps where a consonant sound is missing one of the two complementary series. To meet this need, there are two superscript letters in the Khmer script ៉ and ៊ that can go on top of a base consonant letter to switch its vowel series to the other series. ៉ switches the vowel series to Ą Series, and ៊ switches the vowel series to Ǫ Series.

Series-Shifting Consonant Letters

Note that the letter ប can take both ៉ and ៊, due to it originally being an ancestral p in Indic as well as being repurposed in its base form to cover glottalized /ɓ/.

Some sonorant letters can also take an h in front of them to switch their vowel series from Ǫ Series to Ą Series, as an etymological spelling.

There are also some other supplemental consonant letters that utilize an <h> letter in front of another letter to transcribe sounds that entered Khmer through modern loanwords.

Supplemental Consonant Letters

Note that the usage of ហ្វ for transcribing /f/ means that this glyph does double duty, being used for /ʋɑː/ originally, but in modern loanwords (from French, English, etc.) indicating /fɑː/ instead.

Rimes

For every rime diacritic, learners must remember the vowel separately for the Ą Series consonants and the Ǫ Series consonants (or when there is no difference). As mentioned above, with no rime diacritic, the default rime for the Ą Series consonants is /ɑː/, and the default rime for the Ǫ Series consonants is /ɔː/.

Basic Rimes

Khmer is very particular with its vowel distinctions, so learners should take the time to practice them with native speakers.

Rimes with Coda

Typically, vowels with glottal stop/null initial consonant are attached to អ, but there are also independent vowel letters that are used mainly in spelling Indic words. The exact rime used for these letters can vary by word, so learners must learn them with each vocabulary item.

Independent Vowel Letters

ឨ is no longer used in modern Khmer.