You could use either in your example, with "have to" sounding more imperative, as in something that needs to happen NOW. If you wanted to make it a general maxim, you would use the present tense, and also remove the definite article in front of "learners".
So for your example, using the present tense would mean that some specific, lazy students need to buckle down RIGHT NOW to finish their homework (for example, because it's almost bedtime and the homework is due in the morning). Using the future tense would mean that those same lazy students need to buckle down in the (near) future (maybe the homework is due in a couple of days). And if you wanted it to be a general fact, you would say "if students" with the present tense.
Another example might make this clearer:
If the children want to have dessert, they have to eat their
I would understand this sentence to mean that we are speaking about specific children, and they need to eat the vegetables that are in front of them right now if they want to have dessert after the meal that we are currently eating. Imagine a family sitting at the dinner table, and one parent talking to the other so that the children can hear and be warned.
If the children want to have dessert, they will have to eat their
Again, we are talking about specific children and a specific meal, but in this case, the meal may not yet be taking place (probably it's the next meal featuring dessert).
If children want to have dessert, they have to eat their vegetables.
Here, we're talking about a general rule: eating vegetables is required for any/all children to get dessert.