Translation:My dear, it would be better if you put on the sweater.
You're right that at times articles are used differently but in this case a literal translation does work. You could also say 'un maglione' in Italian to refer to just any sweater but telling somebody to put on 'the sweater' or 'il maglione' indicates that there is one on hand :)
Maglione = it's a heavy sweater (usually made of wool) that you usually wear during winter. Maglia is more generic and it may indicate a lighter sweater, a shirt, or even a t-shirt. Maglietta = t-shirt, or a shirt you would wear during the summer.
Basically you can associate the heaviness of the shirt with their suffix. Maglione = big shirt -> heavy shirt Maglia = shirt -> light shirt Maglietta = little shirt -> lighter shirt
This exercise is difficult because it contains three sources of confusion. Firstly the imperfect subjunctive which cannot be literally translated without totally contorting the English language. Secondly the Italian use of "mi metto il maglione" for "I put on my sweater" (this is why the definite article is used here). Thirdly maglione / maglia is difficult to translate as there is no consistent translation in English (ask an American or English person what a T shirt is....try Googling it!). Having said that, if you can overcome these challenges your Italian (and English) will definitely improve.
Thanks. acqualinda pointed me to this great explanation of the subjunctive. http://blogs.transparent.com/italian/2009/10/page/4/ Point 4 applies here.
That page has moved. It's now at http://blogs.transparent.com/italian/il-congiuntivo-part-4-%e2%80%93-imperfetto/
It explains that the imperfect subjunctive is used following the conditional +che construction for certain verbs including the "essere meglio" used here.
gianberto: Nothing wrong at all. 'A sweater' implies 'your sweater'. My comment would be about DL's choice of 'my dear' which i think is too literal. It sounds like a parent speaking to a child and 'my dear' sounds too formal. I'd say "honey..." which many parents use when speaking directly to their children.
While technically correct, italian like other languages, German e.g., prefers the definite article over the possessive adjective -- il vs il tuo -- with parts of the body and articles of clothing, when the context is obvious. Inclusion of the possessive is certainly not incorrect, but its use isn't necessary to warrant the English translation DL's given. It's never a good idea in translating to be too literal or to strive to achieve a word-for-word equivalency. In doing so the translation will often wind up sounding awkward.
Presumably this is reflexive, ie., “mettersi”, but without context is there a way to distinguish between “mettersi” and “mettere” in the subjunctive imperfect? AND if the sentence had said “it would be better if you put the sweater away” would this be translated simply by adding “via” after “ti mettersi”?
In English, the comparative forms are: good -- better -- best
In Italian, the comparative forms are: bene -- meglio -- il meglio
In other words, the superlative in Italian is indicated by inserting the definite artice (il) in front of an adjective or adverb.
So the Italian here means "better", not "best".