necessary

ne-,无，没有，不能，词源同no,-cess,走，离开，词源同cede,concession.即不能走的，不能离开的，引申词义必不可少的，必需的。

necessary

necessary: [14] The original Latin adjective meaning ‘necessary’ was necesse (it was formed with the negative particle ne- from the stem of cēdere ‘yield’, source of English cede, and hence meant etymologically ‘unyielding’). This was subsequently extended to necessārius, and English acquired it via Anglo-Norman *necessarie.
=> cede, concede, proceed

necessary (adj.)

late 14c. "needed, required, essential, indispensable," from Old French necessaire "necessary, urgent, compelling" (13c.), and directly from Latin necessarius "unavoidable, indispensable, necessary," from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" + cedere "to withdraw, go away, yield" (see cede). The root sense is of that from which there is no evasion, that which is inevitable. Necessary house "privy" is from c. 1600. Necessary evil is from 1540s (the original reference was to "woman").

necessary (n.)

mid-14c., "needed, required, or useful things; the necessities of life; actions determined by right or law," perhaps from Old French necessaire (n.) "private parts, genitalia; lavatory," and directly from Latin necessarius (n.), in classical Latin "a relation, relative, kinsman; friend, client, patron;" see necessary (adj.).

1. For her, books were as necessary to life as bread.

对她来说，书就像面包一样，是生活必需品。

2. Now there is a dawning realisation that drastic action is necessary.

现在人们开始意识到必须采取断然的行动。

3. Fry remaining peppers, adding a little more dressing if necessary.

用油炸一下剩下的辣椒，如果需要再加点调味汁。

4. It's only necessary to use just a very small blob.

只需要很小的一滴就可以了。

5. We will use air power to protect UN peacekeepers if necessary.

必要的话我们会动用空军保护联合国维和人员。