Como dizer "Viessem" em inglês

Translate this please.
A súbita, a dolorosa alegria de um espantalho inútil
Aonde viessem pousar os passarinhos.

Explain "viessdm" in English please!

A súbita, a dolorosa alegria de um espantalho inútil
The sudden, painful joy of a useless scarecrow

Aonde viessem pousar os passarinhos.
Where the little birds would come and land.

You used "vissiem" in past tense and present. Which one is right?

Both are right, to my thinking.

Let´s take your narrative:
A súbita, a dolorosa alegria de um espantalho inútil
Aonde viessem pousar os passarinhos.

It´s something that in English can be expressed with "would + infinitive form of the verb.". In English that simply is used to mean actions that repeated in the past. In the same context of "used to", of course.

In Portuguese we could say "vinham" ou "viessem" depending on context, but certainly would be necessary more context to know which one you meant.
So, after the answer is that, it seems, that you meant "vinham", but then there´s a certain intersection in sense.

That is, sometimes "would + infinitive" can be translated into "vinham" and "viessem", depending on the said "action that repeated" in the "used to" context.
In English the verb is not so clear-cut (it depend on the conditions and circumstances), whereas in Portuguese the verbs when read inform the reader almost straight on if it is related to the past or present. Even, so there are cases in Portuguese in which this intersection would be possible, so we have to analyze your sentences on a case-by-case basis.
One gets the hang of it with time.