Because in Italian you always have to put the article in front of the name. In this case you put il because it is a male article. About the plurals: regular male names end in -o and at the plural they end in -i: piatto piatti. Feminine regular names end in -a at the singular and at the plural they end in -e. But there are several exceptions: for example il giornale (m., newspaper) ends in -e but it is masculine singular, and its plural is i giornali, that is regular. But you will help because the name is always with an article, which is ALWAYS BEFORE THE NAME. So remember very well the articles (il, lo m. sing.; la f. sing.; i, gli m. plur.; le f. sing.)
Excuse me, I haven't read the question. However, above there is a grammatical rule, and also in this answer. In Italian there are prepositions: di (of), a(at,to,in), da(from, by, for or since, at), in (in), con (with), su (on), per(to, for, across, along, through), tra, fra(both tra and fra mean between or among)
Sometimes these prepositions become an only form: the preposizioni articolate. They are simply the union of the preposition you want to use and the article of the name with you want to use that preposition. For example: Il cane è sul tavolo(The dog is on the table). Sul is su+il. Sto camminando sulla banchina della strada (I'm walking on the shoulder of the road). Sulla is su+la. Per, tra and fra don't want the article, so you use on of these prepositions at the normal form followed by the article. Con doesn't want the article but two forms can have the article: con+il (becomes col) and con+i (becomes coi). I hope to be clear and helpful, but search on websites these things, surely they are explained in more technical manner
Nel and nello are articulated prepositions, that is to say they are formed by a simple preposition and an article. Simple prepositions which can form articulated ones are di, a, da, in, su.
Nel is formed by in+il and nello is made up by in+lo. If you know how to use definite articles and the differences between them you will be fine.
Here's a table for you.
Still, there's another preposition, "con" (=with), which is irregular. In fact, the only articulated prepositions formed by "con" are "col" and "coi", respectively con+il and con+i. Con+ other articles behaves normally: con+lo, for example, is "con lo" so they don't merge together.
Apart from di, a, da, in, con and su, other prepositions followed by articles just stay separated.
Please, text me if something is not clear or if you have some doubts. Ciao e buona Pasqua.