O.E. cnawan (class VII strong verb; past tense cneow, pp. cnawen), from P.Gmc. *knoeanan (cf. O.H.G. bi-chnaan, ir-chnaan "to know"), from PIE base *gno- "to know" (cf. O.Pers. xšnasatiy "he shall know;" O.C.S. znati, Rus. znat "to know;" L. gnoscere; Gk. *gno-, as in gignoskein; Skt. jna- "know"). Once widespread in Germanic, this form is now retained only in English, where however it has widespread application, covering meanings that require two or more verbs in other languages (e.g. Ger. wissen, kennen, erkennen and in part können; Fr. connaître, savoir; L. novisse, cognoscere; O.C.S. znaja, vemi).
The Anglo-Saxons used two distinct words for this, witan (see wit) and cnawan. Meaning "to have sexual intercourse with" is attested from c.1200, from the O.T. To not know one's ass from one's elbow is from 1930. To know better "to have learned from experience" is from 1704. You know as a parenthetical filler is from 1712, but it has roots in 14c.
像这样演化来的词总会有一些字母不发音，叫做silent letters，下面举一些例子：
名词：
knapsack,
knar,
knave
knawel
knead
knee
knapweed
knick-knack
knickers
knife
knight
knob
knop
knotweed
knout
knock, knocker
knoll
knowledge
knuckle
knur
knurl
动词：
knack
knap
knock
kneel
knell
knit
knock
knot
know
knut
其他：
knuepple
knacker
还有一些其他不发音的字母：
Silent B
B is not pronounced when following M at the end of a word.
climb
crumb
dumb
comb
Silent C
C is not pronounced in the ending "scle"
muscle
Silent D
D is not pronounced in the following common words:
handkerchief
sandwich
Wednesday
Silent E
E is not pronounced at the end of words and usually makes the vowel long.
hope
drive
gave
write
site
Silent G
G is not often not pronounced when followed by an N
champagne
foreign
sign
feign
Silent GH
GH is not pronounced before T and at the end of many words
thought
through
daughter
light
might
right
fight
weigh
Silent H
H is not pronounced when following W. Some speakers whisper the H before the W.
what
when
where
whether
why
Silent H
H is not pronounced at the beginning of many words. Use the article "an" with unvoiced H. Here are some of the most common:
hour
honest
honor
heir
herb
Pronounced H
H is pronounced at the beginning of these common words. Use the article "a" with voiced H.
hill
history
height
happy
hangover
Silent K
K is not pronounced when followed by N at the beginning of a word.
knife
knee
know
knock
knowledge
Silent L
L is often not pronounced before L, D, F, M, K.
calm
half
salmon
talk
balk
would
should
Silent N
N is not pronounced following M at the end of a word.
autumn
hymn
Silent P
P is not pronounced at the beginning of many words using the suffix "psych" and "pneu".
psychiatrist
pneumonia
psychotherapy
psychotic
Silent S
S is not pronounced before L in the following words:
island
isle
Silent T
T is not pronounced in these common words:
castle
Christmas
fasten
listen
often
whistle
thistle
Silent U
U is not pronounced before after G and before a vowel.
guess
guidance
guitar
guest
Silent W
W is not pronounced at the beginning of a word followed by an R.
wrap
write
wrong
Silent W
W is not pronounced with these three pronouns:
who
whose
whom
