Infiammabile vs. combustibile
In Italian, both infiammabile and combustibile relate to things that can burn, but they represent different levels of fire hazard and have distinct everyday uses. The word infiammabile means flammable and is used for substances that easily catch fire at normal temperatures. On the other hand, combustibile means combustible and refers to materials that can burn but usually require higher temperatures to ignite. Additionally, combustibile is very frequently used as a noun to mean fuel.
Infiammabile
The adjective infiammabile describes something that is easily set on fire and will burn rapidly. It is typically used as a hazard warning for highly reactive materials like gases, alcohols, and certain chemicals.
Questo liquido è altamente infiammabile e deve essere tenuto lontano dal calore.
(This liquid is highly flammable and must be kept away from heat.)
Il cartello indicava la presenza di gas infiammabile nella miniera.
(The sign indicated the presence of flammable gas in the mine.)
Evita di indossare tessuti infiammabili vicino al fuoco del campeggio.
(Avoid wearing flammable fabrics near the campfire.)
L'alcol etilico è una sostanza infiammabile da maneggiare con cura.
(Ethyl alcohol is a flammable substance to be handled with care.)
Hanno stoccato materiali infiammabili nel magazzino principale senza le dovute precauzioni.
(They stored flammable materials in the main warehouse without proper precautions.)
Combustibile
The word combustibile acts as an adjective to describe something capable of burning, usually needing a higher temperature to ignite than an infiammabile object. However, its most common everyday use is as a noun meaning fuel, referring to substances burned to produce heat or power.
Il legno è un materiale combustibile che brucia lentamente.
(Wood is a combustible material that burns slowly.)
Il prezzo del combustibile per aerei è aumentato di nuovo quest'anno.
(The price of aviation fuel has increased again this year.)
Usiamo pellet di legno di alta qualità come combustibile per la nostra stufa.
(We use high-quality wood pellets as fuel for our stove.)
Il carbone è un combustibile fossile che causa molto inquinamento atmosferico.
(Coal is a fossil fuel that causes a lot of air pollution.)
Questa plastica è combustibile ma non prende fuoco facilmente a temperatura ambiente.
(This plastic is combustible but does not catch fire easily at room temperature.)
Summary
To summarize, you should use infiammabile when warning about materials that catch fire easily and quickly. You should use combustibile to describe things that can eventually burn, or, more importantly, when you need the Italian noun for fuel.