Education is enlightening.
In this example, is enlightening a gerund or a participle?
From my point of view (which can be wrong), enlightening is a gerund. Can anyone please explain?
In your sentence, enlightening is something called a participial adjective. It is not a participle, which is a verb form; rather, it is an adjective derived from a participle.
We can show that enlightening is an adjective in your example using a very-test:
Education is very enlightening.
We can modify gradable adjectives with very, but not verbs:
*The water is very running. ← ungrammatical
In this example, running is a verb form, so it can't be modified by very.
In a different sentence, enlightening could be a participle:
With funds from Congress, ShareSpace is gradually enlightening the public.
This is clearly a verb form. It takes a direct object (the public), something adjectives generally don't do. It also takes adverbial modification (gradually) but cannot be modified by very:
*ShareSpace is very enlightening the public. ← ungrammatical
Most of the time, if enlightening is used as a verb form, it has a direct object. Your example has no direct object, which tells us that it's probably an adjective form instead.
In another sentence, enlightening could be a gerund:
By investigating and reporting on the many corrupt activities in the St. Louis area, the paper performs an essential role of enlightening the public.
In this example, the subordinate clause enlightening the public contains the verb form enlightening. We can tell it's a verb form because it takes a direct object. It cannot be modified with very, and it is not an adjective; but it also can't take a determiner, can't be modified by an adjective, and doesn't take noun-like complementation (of the public), so it's not a noun, either. It's a gerund, a verb which heads a subordinate clause that is functionally similar to a noun phrase.
Because they're functionally similar to noun phrases, people often claim that gerunds are nouns. They're not, though. How can we tell?
Let's look at another example.
He was expelled for wantonly killing the birds.
In this example, killing is a verb form, head of the subordinate clause wantonly killing the birds. Because this clause is functionally similar to a noun phrase, some people might be tempted to call killing a noun here. But it's clearly not:
Wantonly killing the birds has the form of a clause headed by a verb, not a noun phrase. It is a mistake to say a gerund is a noun simply because the clause it appears in is functionally similar to a noun phrase.
We can derive an actual noun from killing, if we want to. Let's do that:
He was expelled for the wanton killing of the birds.
In this example killing is a gerundial noun, a noun derived from a gerund. How can we tell?
So we can see a clear difference between a gerund, which is a verb form, and a gerundial noun, which is a noun form.
It's true that an -ing form can be a gerund or participle (if we choose to make this distinction), but in your case it is neither. It can also be a derived adjective, a derived noun, or a derived preposition (like during or notwithstanding). As we saw earlier, in your example it is an adjective derived from a participle.
For more information, see The Cambridge Grammar of the English Language, starting on page 81 (verbs vs. nouns), page 536 (verbs vs. adjectives), and page 610 (verbs vs. prepositions).
In this case "enlightening" is a simple adjective derived from "enlighten", i.e. "something that enlightens". So it's a participle that modifies "education".
A gerund is a noun derived from a verb, for example, "I enjoy swimming, but she enjoys sailing," or, "Running is a good form of exercise but it can be hard on the knees."
Because "enlighten" already has a related noun ("enlightenment"), "enlightening" is not normally a gerund.