The words are closely synonymous but not usually interchangeable. I recommend that you notice collocations e.g. instant access, immediate difficulty.

thank you .

The two words can mean the same thing, or they can mean slightly different things. It all depends on the context in which you use the words. Here are some examples and explanations of what is meant. You can use "immediately" to mean "right now", or "at once"; you're talking about a very short interval of time relative to "now". E.g. "I need you to write me a report and I'd like you to start immediately. So drop whatever else you were doing!" You can use "immediately" to mean "right after"; you're talking about a very short interval of time relative to some event. E.g. "Upon receiving the email, she immediately began to compose a reply." You can use "instant" to indicate that a process can be carried out in a short interval of time. Strictly speaking, "instant" implies that zero time elapses, but this is of course an exaggeration. For example, "instant coffee" (the coffee is ready "instantly", or "in an instant"), "instant mashed potatoes", "instant messaging" (your message is seen "the instant you type it"). You can use "instant" to mean the same thing as "right now" or "at once". E.g. "I need you to write me a report and I'd like you to start this instant. So drop whatever else you were doing!"

Instant means something happening right away, as does immediately. However in a sentence immediately is the word you would use to signify something happening right away. You would not use instant in a sentence really.