Italian, Spanish and Portuguese have more than the fact that they are all neolatin languages in common; they are also what we call null subject languages. That means in some cases, sentences which don't actually have a subject.

Due to that, speakers of these three neolatin languages - unlike the French, which is not a null subject language - struggle to add "it" as the subject of sentences where the pronoun doesn't actually refer to a thing/animal mentioned before (here is a list of speakers of other languages, such as Arabic and Chinese, who might too). In languages such as these, not only is it ok to omit a pronoun and use a conjugated verb alone; we also have sentences which do not have a subject at all. 

Let's take the weather as an example. Suppose a Brazilian, a Portuguese, a Spanish speaker or an Italian sees this:

Here's what we'll probably think/say:

English: "It's snowing!"

Spanish: ¡Está nevando! / ¡Nieva!

Portuguese: Está a nevar!

Brazilian: Está nevando! / Tá nevando!

Italian: Nevica!

Have you noticed the issue? To us, it is perfectly acceptable to use just the verb alone, or be + gerund. After all, to speakers of these languages, subject pronouns have to refer to something or someone. As we learn in school: they substitute a noun. Therefore, adding it just for the sake of having a subject doesn't come in naturally nor does it feel necessary; it's actually really weird. I've had my Brazilian students say things like the following sentences:

- I think is good/difficult/interesting...

- Is necessary to choose... (affirmative sentence)

- I find is great

- Is correct. / Is correct?

- I'm sure is great.

- Is perfect for me.

I am confident if you teach speakers of the other two languages you've come across similar errors too, so what I did for my learners might help you:

1. I gave this class a highly motivational name =)

2. I assigned a pre-class task where they needed to translate sentences from Brazilian Portuguese into English. These were all based on previous mistakes they had made before, so I knew they would translate everything incorrectly and I would have a point. You will see the sentences feature other mistakes too, but I didn't delve into them too much as I wanted to focus on using "it" as a subject.

3. I introduced the topic in class by showing them these were all wrong, which some of them couldn't understand why. I then told them it was because Brazilians speak a null subject language just to make them curious. So I showed them subjectless sentences in our language and said English would never admit that. 

4. After that, I moved to adding "it" to every one of the wrong sentences in a different color, here are some board notes from a class with some great examples one of my students helped me to provide). This was the "AHHHH, now I get it... you're adding a word that has no meaning and I have to do it too. WHY, Teacher? "moment of the class; I was very pleased.

5. At this point, some of them started thinking it = this, so I also wrote "I think it's great" and "I think this is great" and told them the two sentences did NOT mean the same. We worked on that too.  

6. In order to verify understanding, I started making them talk about their opinion on different topics and also about the weather; loads of problems arose and as we worked on further clarification, I could tell they finally realized what their problem was. 

7. For homework, I assigned a reading task from which I removed "it" ten times. They were supposed to read the text and put them back. 

I am really pleased with the results, and I think it was much more effective a strategy than completely ignoring their first language and focusing solely on what I wanted them to do, but not really on the reason why they weren't doing it right. And now, because I speak a bit of Spanish and Italian, I am sure speakers of these languages can benefit from the idea too. 

=)