Uso del verbo stare in italiano / Use of the verb "stare" in Italian
The other day I reviewed an Italian grammar book. Frankly speaking, the grammar in Italian seems to be a hell lot worse than in French or Spanish, because it combines both of them (such as the formation of compound past tense from French, and the concept of essere [ser] and stare [estar] from Spanish), plus its own particular style (e.g., rules of definite article based on orthography; or the use of both definite article and possessive pronoun to describe a personal belonging)! :(

Then, it comes to the concept of "stare" in Italian. In Spanish (and probably Portuguese), the question regarding the differences between "ser" and "estar" has been raised a number of times by various members. Unfortunately, similar question has not been raised in terms of Italian (at least based on the searched results). Now I have some concerns regarding use of "stare" in Italian.

I would like to know, if one wants to express:
1) a temporary condition (such as "to be sick" or "to be a shop assistant [commesso, -a] as a temporary work position");
2) an emotional feeling (such as "to be happy / sad / angry");
3) the current location of a person / animal / object (such as "to be at the door / under the table / in the bus", etc.);
which verb ("essere" or "stare") should one use?

In Spanish, the answers of the above question is quite clear to me (all as "estar"). However, in Italian, I am not fully sure of the respective answers. Hence, I fully appreciate your precious answers and any constructive suggestions. Many thanks for your kind attention.Dear Andrea: Thank you very much for your precious time, your great help and all the information. Let us wait for more answers and suggestions before closing the matter. Meanwhile, I sincerely wish you a very nice day.

Caro Andrea, Grazie mille per il tuo tempo prezioso, il tuo grande aiuto e anche tutte le informazioni. Aspettiamo più risposte e suggerimenti prima di chiuedere la questione. Intanto, ti desidero sinceramente una molto buona giornata.

Question

Uso del verbo stare in italiano / Use of the verb "stare" in Italian
The other day I reviewed an Italian grammar book.  Frankly speaking, the grammar in Italian seems to be a hell lot worse than in French or Spanish, because it combines both of them (such as the formation of compound past tense from French, and the concept of essere [ser] and stare [estar] from Spanish), plus its own particular style (e.g., rules of definite article based on orthography; or the use of both definite article and possessive pronoun to describe a personal belonging)! :(

Then, it comes to the concept of "stare" in Italian.  In Spanish (and probably Portuguese), the question regarding the differences between "ser" and "estar" has been raised a number of times by various members.  Unfortunately, similar question has not been raised in terms of Italian (at least based on the searched results).   Now I have some concerns regarding use of "stare" in Italian.

I would like to know, if one wants to express: 
1) a temporary condition (such as "to be sick" or "to be a shop assistant [commesso, -a] as a temporary work position"); 
2) an emotional feeling (such as "to be happy / sad / angry"); 
3) the current location of a person / animal / object (such as "to be at the door / under the table / in the bus", etc.); 
which verb ("essere" or "stare") should one use?

In Spanish, the answers of the above question is quite clear to me (all as "estar").  However, in Italian, I am not fully sure of the respective answers.  Hence, I fully appreciate your precious answers and any constructive suggestions.  Many thanks for your kind attention.Dear Andrea:  Thank you very much for your precious time, your great help and all the information.  Let us wait for more answers and suggestions before closing the matter.  Meanwhile, I sincerely wish you a very nice day.

Caro Andrea,  Grazie mille per il tuo tempo prezioso, il tuo grande aiuto e anche tutte le informazioni.  Aspettiamo più risposte e suggerimenti prima di chiuedere la questione.  Intanto, ti desidero sinceramente una molto buona giornata.

Answer

2) emotional feelings 
"essere" is the correct verb 
"sono felice" 
"sono triste" 
"sono arrabbiato" 
"sono innamorato" 
"sono turbato 
"sono preoccupato" 

As you can see, the rule helps you in these cases too :) 

3) current location of person/animal/object 
this item is the worst to explain. 
The difference between "stare" and "essere" could be minimal due to a large interchargebility 
"Il gatto è/sta sotto il tavolo" 
"Io sono/sto sulla porta" 
"Io sono/sto sull'autobus" 
"Le chiavi sono/stanno nel cassetto" 
"Io sono/sto al mare" 
The most used form depends on local/regional habits too... but in some few cases only one verb is correct. As I don't have a method to teach it, I think it's better to not complicate things anymore. For the moment consider "stare" and "essere" interchargeble when they point a location. 
I'm also waiting for a professional teacher answering your question in order to have a method to best explain this matter to my foreign friends :). 
in the meantime I hope my answer will be helping you 

Ciao 

Andrea

Answer

Hi mitong,
I understand your concerns.
Your question is about one of the biggest problem that who wants to learn italian must deal with.
This is beacuse "essere" could mean some different things.
"essere" is for sure the translation of the english "to be" but also of "to stay" which can be directly translated with "stare".
In some cases "essere" is also the translation of "to feel" which can be directly translated with "sentire". 
So this matter is quite complicated and it is going to be very hard to work on it. 
In some cases "stare" and "essere" are interchargeble and in other ones they aren't. 
That's why it is a lot of confusion for foreigners.
Other difficulties come out for british people beacuse in italian we use the ausiliar "stare" instead of "essere" ("to be") to construct the continuous forms of verbs:

- I'm looking for a job"
"Io sto cercando lavoro"
and not
"Io sono cercando lavoro"

It's a mess :)
I know it.
It's a very difficult to explain and the only thing we can do is waiting for a professional teacher. I'm native and I spontaneously choose the suitable verb but I don't have a method to teach it.

Anyway I can be of service following your numbered list.
Read below.
Hope this can help you

Answer

1) a temporary condition (es. to be sick)
in this case you can use "stare". 
- How are you? I'm sick/I'm fine 
"Come stai? Sto male/sto bene" 
But your physical condition can be also described by the use of "essere". Examples: 
- I'm tired 
"Sono stanco" 
- I'm sick 
"sono ammalato" (before we translated "sto male" too) 

ooooooooohhh, look at the last example!!!! 

It's suggesting us that whatever the condition a general rule is possible: 
We must use "essere" before nouns and adjectives while we must use "stare" before adverbs. 
This rule is convenient to do practise with "stare" and "essere" and it works in most of cases even though you'll find out some very rare exceptions. Example: 
- I'm comfortable 
"sono comodo" and "sto comodo" could be interchargeble but, as the previous rule wants, "sto comodamente" is correct and "sono comodamente" is not ("comodamente" is an adverb) 

Regardind your second example "to be a shop assistant": 
we don't use "stare". We use "essere" or the more suitable "fare" (because the meaning is to MAKE a job) 
"Io faccio il commesso" 
or 
"Io sono un commesso" (in this case the preavious rule is good again:"commesso" is a noun) 

I'm more and more sure about the usefulness of the rule