English is a very flexible language and you can build on your vocabulary to make new words.
One way of doing this is to add prefixes (such as dis, pre or co) before the word.

Here's a list of common prefixes with their meanings and some examples.

anti (= against)
antibodies, anti-social

auto (self)
autonomous, autobiography, automobile

bi (= two)
bicycle

co (= with)
cooperate, coordinate

contra (= against)
contradict, contravene

de (= remove)
deregulate, deselect

dis (= not)
disappear

il (= not)
illegal

im (= not)
immaterial, immature

inter (= between)
international

mis(= badly/wrongly)
misinform, misbehave, misunderstand

multi (= many)
multinational

non (= opposite)
non-profit

out (= more than)
outperform, outdone

over (= too much)
oversleep, overwork

post (= after)
postpone, postnatal

pre (= before)
predict

re (= again)
rewrite, relive

sub (= under)
submarine

super (= higher/improved)
supermarket

trans (= across)
transatlantic

uni (= one)
uniform

under (= not enough)
underpaid, underfed

Word Endings

You can also make new words from the words you already know by using different endings. For example, "The person who employs me has a fast car". You can make this sentence simpler, by replacing "the person who employs me" with "my employer". This gives you "My employer has a fast car."

In English you can make nouns from verbs (to employ gives employer and employee). You can also make verbs from nouns or adjectives: government gives to govern, modern gives to modernise and so on. Learning what endings you can put on words means you can expand your vocabulary and say what you mean more easily.

Here are some common word endings:

Nouns

-er /- or: a person who does something
adviser / advisor, teacher, learner

-ian
optician, mathematician

-ment: result of action
improvement, advancement

-ism: name of system or belief
realism, optimism

-ist: the person who believes in the system
realist, optimist

-ion
confusion, apparition

-ness
happiness

-ship
leadership

-ence / ance
permanence, appearance

-acy
lunacy

-age
marriage

-ity
annuity

-y
photography

-cy
fluency

Verbs

-ify
falsify, modify

-ise
modernise

Adjectives

-ic
idiotic, periodic

-ful
awful, wonderful

-able / ible
comfortable, terrible

-proof / resistant
waterproof, childproof, fireproof

-free
alcohol free beer, nuclear free zone

-less: without
hopeless, childless