Ability
Can expresses ability, and in general, could is the past form.
- She can diagnosis patients well.
- When she was a child, she could diagnose her teddy bear well.
Other forms of can are expressed by the phrase, "be able to."
- I have been able to understand this prescription.
- She will be able to go to surgery at 8 pm.
- Being able to study oncology has really helped my patients.
- I would love to be able to do that.
To express a past ability on one particular occasion, we do not use could. Was were able to or managed to are used in this situation.
- Were you able to find radiology okay?
- Last week, I wasn't able to read many patient charts.
- He was able to figure out the treatment plan this morning.
We use could when we are describing general past abilities or habits.
- When I was younger, I could run a lot faster than I do now.
- When my metabolism was faster, I could eat pie everyday.
- When I was a child, I couldn't understand the meaning of this word.
Advice
Should and ought to are used to express mild advice. Remember, ought to is the only modal that needs the word "to" before the verb. The negative is "ought not to," but its use is rare.
- You really ought to study more.
- He shouldn't be so careless.
To give strong advice, we use had better. Usually, this means "If you do not do this, something bad will happen." Although we use "had," this does not have a past meaning.
- If you want to do better in class, you had better study and come everyday. (If you don't study or come everyday, you will do terribly in class.)
- She had better call me. (If she doesn't call me, I will be angry.)
The negative of "had better" is "had better not."
- I had better not eat this. (I am allergic.)
- You had better not do that again. (You will get in trouble.)
Exercise
Using what you have learned about ability and advice, complete the activity.