Spanish „a/al“ with a second infinitive

[Constantin]

My today‘s question has to do with a second infinitive in Spanish, introduced sometimes by „a“, sometimes by „al“. I came across this „problem“ while reading an unknown poem by a poet cuyo nombre no me acuerdo.
Example No. 1 is about „a colgar“. No.2 about „al contemplar“.
Is number 2 a kind of a hidden gerund, could „ al contemplar“ be replaced by
„ contemplando“ ? Thank you.

1.Volverán las oscuras golondrinas/En tu balcón sus nidos a colgar.

2. Pero aquellas que el vuelo refrenaban/Tu hermosura y mi dicha al contemplar.

[Rusty]

The English gerund acts as a noun. Its Spanish equivalent is the infinitive.
I like swimming. = Me gusta nadar. In English, we have a full infinitive and a bare infinitive. The «a colgar» in your first stanza is the equivalent of an English full infinitive.

The Spanish gerundio is equivalent to the English present participle, which is a verbal that also ends in 'ing', like the English gerund. But unlike the gerund, which always acts as a noun, the participio is used as an adjective and, by the name gerundio, it is used to form the second part of the progressive/continuous tense.

Another way to use the infinitivo is with a preceding el. This, of course, also acts as a noun. The ending of your second stanza is an example of this, coupled with a preceding 'a' to produce the equivalent of an English full infinitive.

So, the "hidden gerund" is actually an alternate full infinitive, using English vernacular. You can't substitute the adjectival participio.

[Constantin]

El infinitivo puede ser sustantivo y verbo a la vez en español !
This rule was new to me.
Thank you for the complexity of your reply. I am overwhelmed by it.