"Estas bele" vs "estas bela" (or "beli")

Question

I sometimes saw adverbs instead of adjectives being used to describe things, like for example in estis bele ke vi povis alveni. Why wouldn't one use bela instead? After all, in the sentence you are characterising something ("ke vi povis alveni"): the fact that you could come is nice, so bela.

If I had to guess, I'd say that it is because what is being characterised in this sentence is the verb estis instead, and for verbs you should use adverbs, not adjectives. This is a rule easy to follow for other verbs, for example for sentences like li kantis bele. But, when the verb is esti, like in the first example, what is the rule?

Answer 1

The bele here is actually describing the entire sub-phrase (ie, ke vi alvenis), not the word estis. When you are using a word to describe a sub-phrase you should use the -e ending instead of -a. The same applies to describing an infinitive (verb ending in -i) because these are understood like a sub-phrase. This is different from English which uses an adjective instead of an adverb (“nice” instead of “nicely”) as you pointed out. Arguably the Esperanto way is more logical, even if it is a little confusing for English speakers.

Some examples:

Estas malfacile grimpi monton

Here the malfacile describes an infinitive (grimpi monton) so it needs to be an adverb.

Estas malprobable ke mi iĝus prezidanto de Usono

Here the malprobable describes the subphrase (ke mi iĝus prezidanto) so it also needs to be an adverb.

The grammar section of Lernu has some more examples. Look for the paragraph beginning with “We use an adverb as a predicative instead of an adjective, if the described subject is an infinitive or a subordinate clause”