It's easy to get confused about secondo and a seconda di. They sound almost the same, their meanings are similar, but they're used in different circumstances, and have different rules.
In addition to meaning "second," secondo is an adjective used as a preposition to mean “according to” or “on the basis of.” It is usually used subjectively, to express an opinion, what someone thinks.
In the following example Marika is asking our opinion, what we think.
Dove cade l'accento secondo te?
Where does the accent fall in your opinion?
Where do you think the accent falls?
Caption 44, Marika spiega - Gli accenti - Part 1Play Caption
It’s useful to be familiar with this turn of phrase, because rather than saying penso che (I think that), which takes the subjunctive and requires a relatively complicated sentence structure, it’s easier to say secondo me (in my opinion). In this case there is no change to the verb. It’s important to remember, however, that secondo must be followed by an object: a proper noun, noun, or pronoun.
Secondo me, pioverà stasera.
I think it will rain tonight.
You can also say:
Secondo le previsioni, pioverà stasera.
According to the forecast, it will rain tonight.
Secondo is also used to mean "in compliance with."
Dovremmo guidare secondo il codice della strada.
We should drive in compliance with the traffic laws.
Similar in meaning is the curious compound preposition a seconda di (depending on, according to). With a seconda di we can refer to facts or conditions, not opinions. In contrast to secondo, we need three elements: the preposition a (in, to, at), the main preposition seconda, in the feminine, and another preposition di (of).
Following this compound preposition, there may be a noun:
Il codice della strada cambia a seconda del paese.
The traffic laws change depending on the country.
Marika uses this compound preposition in her lesson on accents.
E sì, parliamo della corretta pronuncia delle parole italiane a seconda dell'accento.
Oh, yes. We'll talk about the correct pronunciation of Italian words according to the accent.
Captions 5-6, Marika spiega - Gli accenti - Part 1Play Caption
She could also have followed a seconda di with an adverb, like this:
Parliamo della corretta pronuncia a seconda di come mettiamo gli accenti.
We'll talk about correct pronunciation depending on/according to how we place the accents.
In English we can say "according to me, him, her, us, you, them," and this is when we use secondo, because it is referring to something subjective, an opinion.
We cannot say a seconda di me or a seconda di Elena.
When we are choosing or deciding something, the choice may be made according to certain factors. That’s when we use a seconda di.
Il prezzo cambia a seconda della destinazione.
The price changes depending on the destination.
Il prezzo cambia a seconda di dove vuoi andare: Roma o Istanbul.
The price changes according to where you want to go: Rome or Istanbul.
An opinion is expressed with secondo:
Secondo me, il prezzo è troppo alto in ogni caso.
In my opinion, the price is too high in any case.
We hope to have cleared up a common uncertainty. Let us know if you have any other questions!