"¡Lo siento mucho!"

You don't use "lo" instead of "yo." You can use the yo if you want to. This is relexive, so the "lo" is the object of the verb.. "Yo lo siento" is "I it feel(I)".

"lo siento" is an idiomatic expression

The reflexives are used with certain verbs as an idiom. Much like Saballama said this translates to "I feel it" while another example of this idiom is "Me gusta" which translates to "It pleases me" which is our equivalent to "I like it".

it makes no sense

Lo usually is short for "this"...i think lol

Nosotros lo sentimos mucho. Nuestro=our

When I tried unsuccessfully to make a phone call in South America, the phone company (Moviestar) gave me a recorded message that began, "Lo sentimos....." So "Lo sentimos mucho," might be the best translation for your sentence.

Here I am very sorry is accepted

because that sentence "I'm sorry a lot" in English would mean that you're "sorry often" as in you are constantly screwing up and offending someone/everyone so you're constantly having to apologize for your actions or behavior.

While the sentence given here on DL "Lo siento mucho!" is meaning that you are very sorry for THIS incident of offense or that you are profusely apologizing. A big big apology! lol.

Two completely different things. So that's why :)

To elaborate on what PaulineAnn said, Lo siento is saying "I feel it", it's an expression of empathy or sympathy, which is the same base idea as in "I'm sorry". I feel it.

Siento here is the yo form of "sentir", to feel, not sentar. The "yo" forms of the verbs are the same.

Lo siento is from the verb "sentir" to feel.

"siento" is also the first person conjugation of the verb "SENTIR" which means:

"to feel sorry/to regret"

I think you're confusing 'sentar', to sit, with 'sentir' to feel. Meaning, long hand, 'I feel for/about you/it/any 3rd person object represented by 'lo'.

I think "disculpe" is the usual translation of "excuse me". It's the same in English, in certain circumstances you could use either (for example if you bump into someone in a crowd), but other times they both have distinct separate meanings and one is more appropriate than the other ("lo siento maté tu gato" sounds better than "disculpe, maté tu gato").

I am very sorry

But it seems that the meanings of these phrases are identical, despite the what the direct translation is.

Just to add another correct translation, "I'm very sorry!" does work.

the lo / la / los etc isn't really relevant to this question - the important thing is "lo siento" is just a phrase which means "I'm sorry". It's a really common and useful expression to know, just learn it as "a thing", and don't worry about what the individual parts mean...

"Siento" is a verb. "Lo siento" might indicate the same thing as "I'm sorry," but the literal translation of "lo siento" is "(I) feel it." So it's more appropriate to say you feel it much than it is to say you very feel it.

Of course many words have multiple meanings, both literal and metaphorical. That's how language works. ;) If you're "feeling blue" you're not actually touching a color. If someone's "running for office" they're not literally zooming from lobby to cubicle. If you have "a lot" of curiosity (or puppies), you don't have an empty plot of land that you've filled with curiosity (or puppies).

But really, it's not much of a stretch at all for "mucho" to indicate various types of "more than."

I wouldn't say "I am sorry a lot" unless I meant that I was frequently sorry. "I am very sorry" or "I am really sorry" sound fine to my native ears, although I don't know what Duo is programmed to accept.