"He is a good boy."
Translation:Ele é um bom menino.
Let me first correct you terminology so as not confuse others. The issue is adjectives that come before nouns. There may be cases I am not thinking of, but for the most part adjectives don't modify pronouns.
In Spanish, the majority of adjectives come after the noun. Certain adjectives like those that quantify always come before the noun. But there are a group of adjectives known as meaning changing adjectives, that change their meaning somewhat depending on whether they go before or after the noun. Bueno/a is actually considered to be one of them, but the meaning change is not really meaningful in English. Essentially adjectives are considered more subjective if they go before the noun and more objective afterwards. Since what is generally somewhat subjective, bueno/a is quite often found before the noun.
Here is a link that reviews the placement of adjectives in Spanish
Oops, you are discussing Spanish on a Portuguese lesson page. I have done that before. "bom" and "boa" are being discussed here. https://www.duolingo.com/skill/pt/Adjectives-I/tips-and-notes This link doesn't go directly there so click on the "Adjectives I " lesson and then click on the lightbulb for the tips and notes.
My understanding is that Portuguese, or at least Brazilian Portuguese, uses subject pronouns a lot more than Spanish and Italian, although less than French. I did do some inquiries about this when I first started the course. I know that Duo does use subject pronouns much more frequently in Spanish and Italian, so I wasn't sure. But my understanding is the first subject pronoun is almost always used. I am still suspicious about the answer I found, so any native input would be helpful.
Not a native speaker, but what I've gathered from the forum is that the subject pronoun can only be dropped if it is clear (from the verb) which pronoun is being dropped. Because the third person-verb (é/são) is used for both the second (você/vocês) and third person (ele/ela/eles/elas) they shouldn't be dropped. From 'É um bom menino' it's not clear if you meen 'He is a good boy' or 'You are a good boy'. The 'abstract it' you mentioned in another reply is an exception I guess.