Avere caldo vs. fare caldo
In Italian, both avere caldo and fare caldo translate to being hot, but they are not used in the same way. The fundamental difference is whether one is describing a personal, physical sensation or the external, atmospheric temperature.
Avere caldo
A1This expression describes the physical sensation of being hot. It's about how a person or an animal feels. The verb avere (to have) is conjugated according to the subject (I, you, he, etc.). It literally means to have heat.
Ho molto caldo, posso accendere l'aria condizionata?
(I am very hot, can I turn on the air conditioning?)
Se indossi quel cappotto pesante, avrai caldo.
(If you wear that heavy coat, you will be hot.)
Il bambino ha caldo perché ha la febbre.
(The child is hot because he has a fever.)
Ieri sera avevamo così caldo che non siamo riusciti a dormire.
(Last night we were so hot that we couldn't sleep.)
Dopo aver corso, il cane aveva molto caldo.
(After running, the dog was very hot.)
Fare caldo
A1This expression describes the weather or the ambient temperature of a place. It is an impersonal phrase, meaning the verb fare (to do/make) is almost always used in the third-person singular (fa). It's about the environment being hot.
Oggi fa davvero caldo, restiamo in casa.
(It is really hot today, let's stay indoors.)
In questa stanza fa sempre caldo, anche d'inverno.
(It is always hot in this room, even in winter.)
Non mi piace quando fa così caldo e umido.
(I don't like it when it's so hot and humid.)
Dicono che domani farà meno caldo.
(They say it will be less hot tomorrow.)
L'estate scorsa ha fatto un caldo record in tutta Europa.
(Last summer it was record-breakingly hot all over Europe.)
Summary
In short, use avere caldo to express that a person or animal feels hot (I am hot). Use fare caldo to state that the weather or a location is hot (It is hot). You feel the heat (avere caldo) because the weather is hot (fa caldo).