The Italian personal pronoun lo is a little bit more tricky than it may seem at first glance. In general, it is translated with it. The English pronoun it is used in three very different situations.
The personal pronoun it is neutral. In other words, all the things are it. It is nominative, accusative and dative and a prepositional pronoun.
It is red. (nominative)
I see it. (accusative)
I give it a try. (dative)
I can live without it. (prepositional pronoun)
An English sentence needs a subject. It needs a subject even in the case that there is none. In this case, an artificial subject has to be introduced.
It is raining.
Obviously this sentence doesn't have a real subject, someone executing the action described by the verb, but in an English sentence you have to introduce a subject.
The third function, and this one is the most important in our context is this one. "It" represents an idea, a sequence of words.
A: Do you think that he has repaired the car?
B: No, I don't think so.
The 'it' represents the whole question (whether he has repaired the car or not).
The question is how to translate these different uses of the personal pronoun it in Italian.
case 1) You don't have neutral nouns in Italian, all the nouns in Italian are either feminine or masculine and therefore, there is no need for a translation of it in this case. The English it used this was must be translated with lo / la / le / gli because in Italian all nouns are feminine or masculine.
case 2) An English sentence needs a subject. If there is none, an artificial subject has to be introduced: it. In Italian normally the subject is omitted, there is no need to put a pronoun in front of the verb.
Lui parla molto <=> Parla molto
Lei parla molto <=> Parla molto
You can omit the personal pronoun lui / lei. Therefore, it is very normal to say
Piove* <=> It is raining
Neva* <=> It is snowing
* There is an equivalent for the continuous form in Italian, but as we haven't talked about until now, we use the simple present. The continuous form would be Sta piovendo / It is raining, Sta nevando / It is snowing.
As the subject of the phrase normally is not mentionned in Italian, there is no need to introduce it artificially in the case that it doen' t exist.
case 3) In this case an equivalent for the English it is needed and very urgent because were often we have situation where we want to represent an idea, an ensemble of word, with a pronoun.
A: Credi che va venire? (Do you think that he will come?)
B: No, non lo credo. (No, I don' t think so. / No, I don' t think it.)
But lo can never be the subject of a sentence.
If the abstract idea is to be explained with a relative clause we use that what in English.
That what he said yesterday is true.
This construction is somehow complicated. That represents the idea and that idea it described with a relative clause and the relative pronoun of the relative clause is what. In Italian the construction is similar.
That what he said today was true.
Ciò che lui ha detto oggi è vero.
Ciò is only used in combination with relative clause. If we want to represent an idea without relative clause, we have to use the demonstrative pronoun questo.
This is a table.
Questa è una tavola.
È una tavola.
This is strange.
Questo è strano.
summary: The abstract idea can be the direct object of the sentence. In this case, lo is to be used. The abstract idea can be described by a relative clause. In this case ciò is to be used. The abstract idea can be the subject of the sentence without being described by a relative close, in this case questo is to be used.