Learn Portuguese (Portugal): O valor específico, não o sei descrever. - The specific value, I do not know how to describe it.

This sentence uses left-dislocation to emphasize the topic. The direct object (O valor específico) is moved to the very front of the sentence. When this happens in Portuguese, you must use a resumptive clitic pronoun (here, o) in the main clause to point back to that fronted topic. It literally translates to 'The specific value, I don't know how to describe it.'

Why is the pronoun o placed before sei rather than attached to the end of it?

The negative word não acts as a magnet. In European Portuguese, certain words like negative adverbs pull the object pronoun to the position immediately before the conjugated verb. Without the não, the pronoun would normally follow the conjugated verb, as in sei-o descrever.

Would it be correct to attach the pronoun to the infinitive instead, like não sei descrevê-lo?

Yes, absolutely! When you have a conjugated verb followed by an infinitive, you generally have the option to attach the object pronoun to the very end of the infinitive. Because descrever ends in an R, you drop the R and add an L, making não sei descrevê-lo a perfectly valid and common alternative to não o sei descrever.

Since we are referring to a specific noun (o valor), why use sei instead of conheço?