17The morpheme used to express indefiniteness in English has two forms-a before a word that begins with a consonant and an before a word that begins with a vowel. The variant forms of a morpheme are called its .
(A) free morphemes
(B) bound morphemes
(C) allomorphs
(D) affixes

(A) (B)

Free morphemes

【定義】(可以自己單獨存在且有意義的)

Morphemes that can stand alone to function as words are called free morphemes. 

They comprise simple words (i.e. words made up of one free morpheme) and compound words (i.e. words made up of two free morphemes).

【舉例】

Simple words : the, run, on, well

Compound words : keyboard, greenhouse, bloodshed, smartphone

Bound morphemes

【定義】（不能自己存在，一定要跟別人合在一起）

Morphemes that can only be attached to another part of a word (cannot stand alone) are called bound morphemes. 

【舉例】（就像我們常用的字根字首！）

pre-, dis-, in-, un-, -ful, -able, -ment, -ly, -ise

pretest, discontent, intolerable, receive

小試身手（紅色free morphemes ; 藍色bound morphemes）

(1) encountered = en + counter + ed

(2) assignment = assign + ment

資料來源

(C) allomorphs 

【定義】

A morpheme is indicated as one or more morphs (surface forms) in different environments. These morphs are called allomorphs. 

【舉例】

資料來源：https://www.slideshare.net/dr.shadiabanjar/allomorphs-dr-shadia-yousef-banjar?next_slideshow=1

Free morpheme 自由詞素: 不可拆分、可獨立成字（dog, cat)