Except for very few words as: glicine, glicerina, anglicano and a few others, the italian sound "GLI" doesn't have an English equivalent.
IMHO it is more useful to listen and try to repeat the "GLI" sound inside the words (coniglio, famiglia, maglietta, paglia, foglia, moglie, sbaglio)
You'd probably use "Gli ombrelli sono dentro il ristorante" for that.
Dentro is used to specify inside. nel/nella/nello/etc is used for a more general "in". I don't know why... Same reason we have both in and inside in English I guess. I can't think of a situation where one is right over the other off the top of my head but I'm sure there are situations where that's the case.
Okay, nel is actually 'in the" because it's the words in + il, just as nella is in + la, but you knew that right?
I was trying to search for some differences and came across this. "Inside" is slightly different because it conveys being enclosed (in the box vs inside the box--which is more than likely closed). Also, I came up with this example where "inside" might be weird to use: if you say "I live inside the USA" instead of "I live in the USA," (unless speaking about borders which gets to the whole being enclosed thing). If you used it with a city it might convey depth so "I live in New York City" vs "I live inside New York City." The latter (inside) seems like the person lives really well within the city or never leaves it.