"It" is an event that is planned for a specific day and time at a specific place. You want to talk about the weather at that place and at that time. That is specific and requires the definite article "the". "Is that event going to happen or not?" "It depends on the weather." I would omit "the" in the following example. Different people might think about different factors before making a decision to go ahead with an event. Weather is the most important factor for the key decision maker. "It does not depend on money. It does not depend on availability of special guests. It does not depend on coverage by television. It depends on weather."

In English the weather means climatic changes but there is also a word "whether" this is used for "if" Did you mean weather or whether I can't understand

You say ‘the weather’ because it refers to the particular weather at that time and place.

Hello. As far as grammar rules go, I do not know the exact answer but I can explain a bit. "It depends on weather." sounds strange and incomplete, "weather" needs to be definite in this case. There are similar constructs to this, such as "It depends on the situation" However there is another scenario using the word "whether" which is pronounced the exact same way as "weather." "Whether" is a conditional and you do not use "the" before it. -"Do you want to go to the park tomorrow?" -"That depends on whether or not it rains tomorrow, I don't want to go if its rainig." I hope this helps.

"a" is the indefinite. Sorry, the accent was from my frwnch keyboard. My native language is English :)