"Afternoons on the beach are fun."
Translation:Las tardes en la playa son divertidas.
Divertido/a(s) is generally considered to be a characteristic/property of something, rather than a state something is in, so ser is generally the verb you see paired with it (using the son conjugation in this case).
Están would only potentially make sense in this context if some specific days on the beach were being referred to as part of a state, in other words having emphasis on being fun "in the moment". In that case you probably wouldn't use Las though, more likely it would be something like "Estas tardes en la playa están divertidas." I think son is still more likely even in this context though.
A better example for where an adjective is sometimes used with both verbs is food. You'd use ser to say that a (type of) food is good/delicious/whatever in general, but estar to put the emphasis on the specific preparation (state) of a food that you're currently eating.
So try and remember estar = state, and that ser is used for things regarded as characteristics which can include those that are subjective or non-permanent. Hopefully it will help decide these sorts of cases until you eventually build an intuition through exposure/practice.
This is a great and thorough explanation, thanks! What you say makes sense... However...I still feel "son" is not logical to one's opinion of an afternoon on the beach. I get that here "son" indicates an inherent overall opinion of a beach afternoon, but that opinion may not be permanent. Let's say, for example, something bad happens to me on the beach which forever soils my overall opinion of an afternoon on the beach. The inherent opinion/state changes. I am a beginner Spanish learner, so I admit to understanding the nuances of the lamguage far from fully, but still...To me, not being able to use both 'estan' and 'son' here seens...inflexible.
It is not so great an explanation as you are still confused. You would not use están here.
How you are and where you are use a form of estar. Use estar also with other verb participles to form certain tenses. Permanent vs. temporary is a bad way to look at most uses of these verbs. There are a few adjectives which have different meanings based on which verb is used. This is not just one afternoon. This is a generalization. Spanish is inflexible about this. You have to use one or the other and there are rules when to use each, An opinion might use a completely different tense in the subjunctive mood, but we will learn that later.
You may not agree with this generalization, but that does not change its form. You would have to add extra words into the sentence to use "están". There is no emotion or bad experence mentioned in this sentence. This is not about you or I and how we feel about afternoons. This is a general statement that you may disagree with. It may be that someone had said that "Mornings at the beach are foggy and cold." So now we are talking about afternoons being a better time to go there. Temporal things use "ser". "Fun" is not a state or condition, but rather it is a characteristic. You could be happy, because you are having fun. Your happiness is an emotional state or condition that would use a form of estar.
I just edited the post you replied to since I think one or two parts may have been misleading.
Whether it's an opinion or whether that opinion could change is irrelevant, that's looking at it from the wrong direction. What's relevant is...
If it makes sense to say that afternoons on the beach in general are "in a fun state" (basically no)
If it did make sense, whether that's the meaning being expressed (definitely not)
Whether, even if those criteria were met, Spanish prefers to regard divertidas grammatically as a characteristic, and therefore prefers son regardless (it strongly does)
When a sentence begins with a noun that is a generality, it needs an article (el, la, los, las) before it. In English, we say "Afternoons at the beach are fun." but in Spanish, because afternoons is a noun, it's, "Las tardes en la playa son divertidas."
The exception is when you are addressing someone directly. For instance, "Mr Sanchez is nice." would be "El Senor Sanchez es simpatico." When addressing Mr. Sanchez directly, you do not need the article. So, "Mr. Sanchez, you are nice." would be "Senor Sanchez, usted es simpatico."
All nouns are either masculine or feminine and many words end in o for masculine or a for feminine. Adjectives must match the gender as well as the number of the noun. “Tardes” is feminine and plural, so it takes “divertidas.” When you don’t remember the gender of a noun, you can look it up in a dictionary. This one has sf in red which stands for sustantivo feminino or feminine noun. https://dictionary.reverso.net/spanish-english/Tardes
Compare: previous exercise - "Los fines de semana en la playa son muy divertidos" vs "Las tardes en la playa son muy divertidas" - it's not "la playa" that determines which form to use - divertidos or divertidas - but rather "Los fines de semana..." which is masculine or "Las tardes.." which is feminine. Glad I wrote this out...I was so confused myself until I did LOL.
All characteristics use ser.
Afternoons on the beach are always fun, not just sometimes. No really, when to use each is more complicated than permanent vs, temporary. https://baselang.com/blog/basic-grammar/ser-vs-estar-the-only-guide-youll-ever-need/
Some adjectives change meaning depending on which verb you use: https://www.thoughtco.com/use-of-ser-or-estar-changes-meaning-3079083
It is the 3rd person plural form required by "las tardes."
If you mean, why "son" instead of "están then it is because divertidas is a characteristic of "las tardes en la playa". Apparently, afternoons at the beach are always fun.