dramatic, drastic, great, large, marked, sharp, significant, substantial
There has been a sharp decrease in pollution since the law was introduced.

modest, slight, small
gradual, progressive, steady
rapid, sudden
corresponding, relative
Fewer houses are available, but there is no corresponding decrease in demand.

general, overall
threefold, tenfold, etc.
5%, 25%, etc.

demonstrate, display, exhibit, indicate, reveal, show
This year's figures show a decrease of 30% on last year.

experience, have, suffer
It is not uncommon to experience a decrease in confidence after a divorce.

report
Half the companies in the survey reported a decrease in sales.

cause, lead to, produce, result in

on the decrease
Marriage is still on the decrease.

decrease from … to …
a decrease from 62% to just under half

decrease in
The new treatment led to a huge decrease in the number of deaths.

decrease of
a decrease of 20%

considerably, dramatically, drastically, greatly, markedly, sharply, significantly, substantially
slightly, somewhat
Spending has decreased slightly this year.

rapidly
progressively, steadily
gradually, slowly

by
Crime has decreased by 20%.

from, to
Average family size has decreased from five to three children.

with
The number of quarrels among children decreases with age.

decrease in number, size, value, etc.
The heart gradually decreases in size.

Decrease is used with these nouns as the subject:
amount, concentration, confidence, consumption, deficit, density, emission, incidence, length, level, mortality, output, percentage, population, productivity, proportion, rate, size, speed, value, velocity, volume, width

Decrease is used with these nouns as the object:
amount, appetite, blood pressure, chance, concentration, cost, diversity, dosage, duration, effectiveness, efficiency, force, frequency, incidence, intake, intensity, level, likelihood, mortality, number, per cent, point, probability, production, productivity, rate, recurrence, reliance, risk, size, stress, temperature, use, voltage, volume, vulnerability, width