Can you explain me, with examples, what the CONGIUNTIVO tense is and how do you form it?

While we use the present indicative tense to talk about certainty and facts, we use the present subjunctive form after a verb or expression of emotion, opinion, desire, uncertainty or a command in the present tense. In other words, the CONGIUNTIVO marks any subjective and personal approach.Most of the Italian verbal tenses you already know (presente, passato prossimo, imperfetto, futuro etc) are all tenses belonging to a verbal tense called “INDICATIVO”. Just like the INDICATIVO, the CONGIUNTIVO has its own selection of verbal tenses (presente, passato, imperfetto and trapassato). The main idea is that we use the CONGIUNTIVO when a statement is not in the main clause, but it’s actually dependant on the main clause. In other words, we tend to use the CONGIUNTIVO is subordinate clauses. MOST TYPICALLY this means in clauses that are preceded by “che”.So we say:Indicative: E' una buon'idea / It's a good idea. This is a FACT.Subjunctive: Penso che sia una buon'idea. / I think it's a good idea. This is just my opinion. Indicative: Mario lavoro troppo. / Mario works too much. This is a FACT.Subjunctive: Ho paura che Mario lavori troppo. / I'm afraid that Mario works too much. In my opinion Mario works too much and this is a problem for me. 
How do I form it?To make the CONGIUNTIVO PRESENTE of regular verbs, you have to remove the ending from the infinitive form of the verb and add -i; -a; -isca accordingly to the conjugation. For example: Cantare > che io cantiPrendere > che io prendaSentire > che io sentaCapire > che io capisca
However, irregular verbs such as ESSERE (to be) or AVERE (to have) have their own forms which result from the stem of the 1st person singular (io) of the present indicative. For example:Essere > che io siaAvere > che io abbia