"These are lakes."
It’s not exactly that it implies invisible is; it’s that this/these have two different kinds of functions in English, which behave a bit differently in Russian.
This/these can be pronouns, in e.g. This is big, These are dogs. The pronoun version always corresponds to это in Russian, regardless of the gender/number of what they’re referring to.
On the other hand, this/these can be determiners, modifying another noun: this cake, these lakes. The determiner version corresponds to это, эта, or эти in Russian, depending on the gender/number of the noun being modified.
So Это человек can mean either This person or This is a person. But Это озёра can only mean These are lakes, with these as a noun — these lakes would be эти озёра, since when these is a determiner for a plural noun, it has to be the plural version эти.
Putting asterisks around text makes it italics, e.g.
Italicise the *third* word gives: Italicise the third word. (Search combinations of “Duolingo”, “formatting”, and “markdown” for more about how this works.) And traditionally, the italic forms of Cyrillic letters are roughly like the cursive forms.
This is a bit font-dependent — most serif fonts have the traditional (cursive-like) italics, but most sans serif fonts use something more like a slanted version of the non-italic forms. On my system, Duo gives a serif font with simple slanted italics — but I guess your system has slightly different fonts, with the traditional italics!
"lake"="озеро" if a gender neutral noun like lake ends in "-о" then the ending is changed to "-а" https://en.wikipedia.org/wiki/Russian_grammar#Neuter_nouns
Sorry no one responded to your question. Inanimate neuter nouns that end in o have the o changed to a for the plural. These ARE the rules you're seeking: