considerable, enormous, great, heavy, real, severe, terrible, tremendous
It's a real strain having to get up so early!

slight
increasing
constant
excessive, intolerable (esp. BrE), unbearable, undue
emotional, financial, mental, nervous, physical, psychological
The mental strain of sharing an office with Alison was starting to show.

be under, come under, feel, suffer, suffer from
Television newsreaders come under enormous strain.
After weeks of overtime, she was starting to feel the strain.

cause, create, impose, place, put
Increasing demand is placing undue strain on services.

ease, reduce
cope with, stand, take
increase

show, take its toll (on sb), tell (on sb) (BrE)
After weeks of uncertainty, the strain was beginning to take its toll.

under the strain
The ice gave way under the strain.
He broke down under the strain of having to work twelve hours a day.

strain on
Losing the business put a strain on their relationship.

a bit of a strain
I found it a bit of a strain making conversation with her.

signs of strain
After three years, their marriage was beginning to show signs of strain.

stresses and strains
the stresses and strains of a long day

take the strain off sb, take the strain out of sth
The Internet takes the strain out of shopping.

bad
slight
back, eye (esp. BrE), muscle, thigh, etc.

be suffering from, have
get
You'll get eye strain if you don't put the light on.

recover from, shake off (BrE)
Gerrard will play if he can shake off a slight thigh strain.

new
mutant
virulent
antibiotic-resistant, drug-resistant, resistant
bacterial, viral
flu, influenza

discover, identify
analyse/analyze, examine, test

strain of
H5N1 is a strain of avian influenza.

hard
You could see he was straining hard to understand.

forward
I strained forward to get a better view.

have to

against
The dogs were straining against the sled.

at
Several men were straining at a rope, trying to move the stalled vehicle.

for
Their ears strained for any slight sound.

under (often figurative)
The company is already straining under the weight of a $12 billion debt.

strain to hear sth, strain to see sth
We had to strain to hear what was being said.

seriously, severely
The dispute severely strained relations between the two countries.

strain sth to breaking point (BrE), strain sth to the breaking point (AmE)
Our public health laboratories are strained to (the) breaking point.

strain sth to its limits, strain sth to the limit

Strain is used with these nouns as the subject:
ear, eye, muscle

Strain is used with these nouns as the object:
budget, credulity, ear, eye, finance, juice, ligament, liquid, mixture, muscle, neck, nerve, patience, resource, spaghetti, voice