Hi Behnam. Yes, it's a bit confusing. I'll try to explain. You can use "a" for either sentence. The word "a" makes the reference non-specific, so "a car" can mean "any car" or "every car" or just "cars in general." It makes sense to say that cars, in general, should all be secure, and it makes sense to say that computers, as a general type of thing, are important research tools. So "a" is fine in either case. The word "the" makes the reference specific. In most cases, using "the" will mean that you're referring to one specific thing, so if you say "the computer," the natural meaning is "this one specific computer, which we're talking about right now." However, under certain circumstances, you can also use "the" to specify a TYPE of thing. In this case, "the computer" doesn't refer to any computer, or even all computers -- it refers (specifically) to the CATEGORY of objects that are called computers. For the most part, you can only use "the" in this way if you are making a broad claim about the properties of a class of things. For example, if you just say "The wolf hunts at night," your reader will probably assume that you're talking about one specific wolf. But if you say "The wolf is an animal that hunts at night," then it's clear that you're talking about wolves as a TYPE of animal. In your example, saying "is an important research tool" makes it clear that we're talking about computers as a TYPE of research tool. If you say "The car must be secure," it sounds like you're talking about one specific car. You could say "The car is a secure mode of transportation." That would be fine. It's clear that "the car" refers to cars as a TYPE of transportation. It doesn't refer to a specific car. It refers to a specific category of things.

When referring to a category of "things" instead of referring to actual "things," the plural without an article is commonly used. (English Grammar in Use 4th edition, Murphy, unit 75) Computers are important research tools. However, the indefinite or the definite article are sometimes used with the same general meaning. A computer is an important research tool. The computer is an important research tool.

When we talk about a specific thing, we use "the". However when we talk about general things we use "a" or "an". "The" is used for things already mentioned/known.