That doesn't work in this case. Googling for "la maglia" gives pictures of a three-quarter sleeve shirt/jersey and a capped sleeve blouse/shirt but also a three quarter sleeve cropped top knit sweater, a sleeveless turtleneck knit top, a long-sleeved lace (and see through) shirt, a shirt with batwing sleeves... that's just the first few pictures. Notice that none of these are t-shirts as suggested by other people in this forum. Searching "un maglione," however, provides all pictures of sweaters.
Congratulations, then, you found out that “maglia” doesn’t (just) mean T-shirt, and that it is more versatile than “maglione.” So it was useful, right? : ) I wouldn’t personally translate “T-shirt” as “maglia,” though there is a little overlap. For a word closer to “T-shirt,” I would use “maglietta.”
You should buy the Oxford-Duden Italian Picture Dictionary. It has not been updated recently so you will not find cell phones and routers in it. It is not as good as the German one. That is the original and the pictures are pictures of German and British trains and cops but it is very useful. It does not just tell you the difference between (for instance) un ombrello and una ombrella, it shows you.
why do I have to use - but only sometimes - American English, when I'm in the UK? We don't use the term "sweater" - it is either a jumper, or a cardigan, but although once they let me use jumper, and said it is correct, this time only a sweater is allowed with other terms - like pantaloni, they let me use the term trousers, while only pointing out, that pants is another correct answer. So how can I get it ALWAYS right, when sometimes they allow a British term, while at other times they claim it's the wrong answer, and only the American term counts as right? At other times
The system automatically accepts common contractions (like he’s, she’s, don’t, won’t), but if you use less common ones (like “woman’s” for “woman is”), they are not likely to be in the system, and in most cases they have not been added by the course contributors. I would avoid them on Duolingo just because more likely than not they have not been entered as a correct answer.
Break up the word for a clue to the meaning. "Lavorare a maglia" refers to the work or action of knitting. So knitwear in general is maglieria. "one" is an ending that means "big." So un maglione is a relatively big or heavy piece of knitwear -- a sweater. By default, then, un maglia, senza "one," is a lighter piece of knitting -- a polo shirt or tee-shirt. That said, there is a lot of overlap in common usage. But I'm a knitter so I'm careful about the distinction.