What is the future tense of the word can?
One way to express it is "will be able to", but what is actually the future tense of "can" ?
The word can comes to us from the Old English cunnan, to know, which belongs to a Germanic-language class of verbs called "preterite-present." These verbs have a preterite (i.e., past tense) inflective pattern in the present tense. In English, the verb is defective, in that it has only the present and past forms (the latter being could) and no future form. The verb is used modally, i.e., as an auxiliary with other verbs to express aspect or mood. In the case of can, that aspect is ability.
English has only two tenses a present tense and a past tense. We often use the modal verb will to express futurity in English. However, there are many other means of doing this, for example by using present continuous, going-to, and even the present simple. We also often use the modal verb can to talk about the future too.
However, we cannot use the verb modal verb can and the modal verb will in the same verb phrase. The reason for this is that we cannot use two modal verbs together. The verb after a modal verb must always be in the plain form (the "infinitive"). However modal verbs have no plain form, so this is not possible. This isn't something special about the verbs can and will in particular, although this example is very commonly discussed. It applies to any of the modal verbs. So, for example, in English, we cannot say:
- *You might must go there tomorrow.
- *You will must leave before the end of the concert.
- *You shall can speak English perfectly when you finish this course.
These sentences are all ungrammatical because they contain two modal verbs.
In answer to the Original Poster's question then. English has no future tense, we need to use special ways of talking about the future. We can use the verb can to talk about the future. We can also use the verb will to talk about the future. However, we cannot use can and will together:
- We can go for dinner after the meeting.
- We will go for dinner after the meeting.
- *We will can go for dinner after the meeting. (ungrammatical)