The difference is that to set fire is to cause something to burn. When something is burning, it is being consumed by fire. Burn (verb): 1. (of a fire) flame or glow while consuming a material such as coal or wood. "a fire burned and crackled cheerfully in the grate" 2. be or cause to be destroyed by fire. "he watched his restaurant burn to the ground" Burn (noun): 1. an injury caused by exposure to heat or flame. "he was treated in the hospital for burns to his hands" 2. consumption of a type of fuel as an energy source. "natural gas produces the cleanest burn" Set fire (idiom): 1. to deliberately cause something to burn. "Someone set fire to the shack"

I can explain this with the following sentence: "I set fire to it, and watched it burn." You set fire to something (ignite it) Fire burns (fire burns paper - which turns to ash) You can also burn your skin with a hot stove/kettle - here there is no fire, but the heat caused damage to your skin, "I burnt my hand on the kettle/stove/fire" hope this helps.

'Set fire to' is igniting/starting a fire. It is not about the entire process of burning, but just the start of it. "He set fire to the house" = He lit/started the fire to the house. We do not know from the sentence if the house is destroyed, only that he started a fire to the house. 'Burn' refers to the entire process, whereby something is consumed or destroyed by fire. "He burnt the house" = He destroyed the house by fire. This means he set fire to it AND the house was destroyed. "He intends to burn the house" = He intends to set fire/start the fire to the house AND destroy it (by fire)

To set fire to something is to start it burning. Burning is the whole process, which in some fires can go on for days and weeks. In conversation, you might say you are going to burn something; of course, you would have to start by setting it on fire, setting fire to it, setting it alight (all identical in meaning, really). These are literal meanings. Both "burn" and "set fire" also have slang meanings. Have a look at https://www.urbandictionary.com/define.php?term=burn https://www.macmillandictionary.com/thesaurus-category/british/to-burn-or-to-start-a-fire https://www.urbandictionary.com/define.php?term=Set%20Fire%20to%20the%20Rain