As a noun:
* The strain of caring for her elderly parents was taking its toll on her health.
* The strain of the heavy weight caused the rope to snap.
* The earthquake put a great strain on the city's infrastructure.
* He felt a strain in his hamstring after the long run.
* There was a strain of sadness in her voice.
As a verb:
* The weight of the box strained my back.
* The long hours strained her patience.
* The company is straining to meet the new deadlines.
* He strained to hear her voice over the loud music.
* She strained her eyes to see in the dim light.
As an adjective:
* He had a strained relationship with his father.
* The atmosphere in the room was strained and tense.
* She gave him a strained smile.
* The doctor said the injury was strained, not broken.
These are just a few examples, and the best way to use "strain" in a sentence will depend on the specific context.