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A helping verb is a verb, such as am, is, was, were, have, has, and had. A helping verb is used as an additional part to a tense, to form it. The helping verbs must be used in the correct combination with the verbs in the other part of the sentence. Some of the helping verbs cannot be used when the verb in the second part of the sentence is in the past tense.
Helping Verbs
| Not used in the past tense | Can be used in the past tense |
| can | could |
| may | might |
| must | had to |
| will | would |
| shall | should |
| have | had to |
| has | had to |
Incorrect: When I looked at my study habits, I decided that I really can work harder.
Correct: When I looked at my study habits, I decided that I really could work harder.
In the example, looked and decided are both in the past tense. Can, therefore is an incorrect choice as a helping verb with work. The helping verb could is correct. Try some other helping verbs from the chart that can be used in the past tense, and all of them could be used correctly in this sentence.
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