Someone came up with a list of roughly 300 words that can be used to convey any basic idea in any language, for the purpose of learning how to communicate effectively in an existing language. I modified that list to make it more suited to constructing languages, like taking out things that may not exist in fantasy worlds (coworkers, offices) and adding about 200 essential words. Hope this helps!

(Tip: If you want a printable 2-column chart of this list, copy and paste this into Word. Select All, go to Insert Table, select Convert Text to Table, and set Columns to 2. Boom! Handy dandy chart for when you don't have your computer.)

Pronouns
I
you she he
they (singular) we
they (plural)
it

Conjunctions that (as in “I think that…” or “the woman that…”)
and or
but because though
so
if

Prepositions
of
to
from
in
at (a place)
at (a time) with
about
like (meaning “similar to”) for (warning, this one has several meanings)
before (also as a conjunction) after (also as a conjunction)
during

Question Words
who what
where
when
why how how much
which

Nouns
everything
something
nothing everyone
someone no one
thing
person place
time (as in “a long time”) time (as in “I did it 3 times”)
friend enemy
woman
man
money
city
country
word
language
food
drink
breakfast
lunch
dinner house
road
store
company
manager
worker job
work
problem
question
solution
idea
life
death
world
space (as in “the universe”)
second minute hour
day
week
month
morning
daytime
night
sky
sun
moon
star
mother, father, parent daughter, son, child
wife, husband, spouse
girlfriend, boyfriend, partner body
head
chest
back
arm
leg
hand
foot
eye
ear
mouth
nose
mind
soul
emotion
joy
anger
sadness
love
hate

Verbs
be
there is
have
do
go
want
can need
think
know
feel
say
like
speak
learn
understand work (as in a person working)
work (meaning “to function”, e.g. “the TV works”)
see
smell
taste
use
should could
believe
practice
seem
come
leave
return stay
give
take
bring
look for
find
get (meaning “obtain”) receive
buy
sell
try
start (doing something)
stop (doing something) continue
finish rest
sleep
wake up
get up eat
drink
happen create (aka “make”) destroy/break
cause
meet
ask (a question)
request
reject reply
mean (as in “intent”)
read
write
listen hear
remember
forget choose decide be born
die
kill
live
change help
hurt
send
study
improve
hope
care

Adverbs a lot
a little
well
badly
only
also
very
too (as in “too tall”)
too much
so (as in “so tall”)
so much
more (know how to say “more … than …”) less (know how to say “less … than …”) as … as … (e.g. “as tall as”)
most
least
better best
worse
worst
now
then
here
there
maybe
always usually
often
sometimes
never
not
today
yesterday
tomorrow
soon
almost already
still
even
enough

Adjectives
the, a (technically articles)
this
that
good
bad all some
no
any many
few
most
other
same
different
enough none
first
next
last (meaning “past”, e.g. “last Friday”)
last (meaning “final”) easy
hard
early
late
important
interesting
fun
boring beautiful
big
small
busy
excited
happy
angry
sad
tired
ready
favorite
new
old
right (meaning “correct”)
wrong
true
false

Phrases hello
goodbye thank you
you’re welcome
excuse me (to get someone’s attention)
sorry
it’s fine (response to an apology)
please
yes no okay
My name is What’s your name?
Nice to meet you.
How are you?
I’m doing well, how about you?
Sorry? / What? (if you didn’t hear something)
How do you say ___? What does ___ mean? I don’t understand.
Could you repeat that? Could you speak more slowly, please?
Well (as in “well, I think…”)
Really?
I guess that
It’s hot. (talking about the weather)
It’s cold. (talking about the weather)
I love you.
I hate you.