Why is "es", not "son", used here?
"Hay cosas que es mejor dejarlas en el pasado."
Given that "cosas" is plural, I would have expected "Hay cosas que son mejor ..."
To use "it is better" here, you'd have to change the structure of the sentence and make "to leave in the past" the subject: "It is better to leave some things in the past".
Literal translation: "There are things that are better to leave in the past"
More natural translation: "Some things are better left in the past"
I'm not sure about grammar here. "Es mejor"isn't an attribute of "cosas". Compare with "Hay preguntas que son mejores que sus respuestas". <- "to be better" is a quality of those questions. However, when you speak about things that are better left in the past, I think that Spanish structure is saying that "es mejor" belong to the act of leaving them in the past.
There are things that it is better to leave in the past. Hay cosas - there are things. (Can be a phrase by itself, although odd with things.) que - a conjunction, putting two phrases together. es mejor (it is better) dejarlos to leave them en el pasado. In the past. cosas isn't the subject of the sentence here, it's in a separate sentence put together with the second sentence by use of the conjunction que. In English, if you leave out the it, the sentence needs are, even though a diagram of the sentence shows two separate independent clauses with an understood subject in the second one. (There are things that are better to leave in the past. If you tried to put in an explicit subject in the second clause using are, it wouldn't work There are things that they are better to leave in the past, explicit subject requires it is) but in Spanish the subject pronoun is implicit in the verb - so singular is the preferred choice.
2019-12-11 I Googled Hay cosas, and the phrases that came up with a verb for the "things" used the plural conjugation. I then googled Hayan cosas and got a few hits for that, as well. However, when I Googled Hay cosas dejarlas, the phrase "Hay cosas que es mejor dejarlas…" appeared repeatedly, without one son. So it appears to be a common phrase in Spanish.
The only explanation I can think of is that there is an implied subject of "it", and the hay cosas is a dependent clause. We could probably rephrase this as, Es mejor dejarlas en el pasado, estas cosas.
That sentence is a composition of other two: 1) Hay cosas. 2) Es mejor dejar las cosas en el pasado. As you can see, "cosas" is the object of two verbs: "haber" and "dejar". It is not, by any means, the subject of the verb "ser". "Dejar las cosas en el pasado" is the subject of the verb "ser"; it is a subject clause. The sentence 2 could be rewritten in this order: [Dejar las cosas en el pasado] es mejor You could ask: "¿Qué es mejor?" The answer would be "Dejar las cosas en el pasado es mejor." So that is the subject.
The reason you are confused is that you are thinking of the English translation: "Some things are better left behind". But that has a different structure. A translation that would mimic Spanish structure more accurately would be: "Some things, it is better to leave them behind."