We’re going to take the adjective “weak” and see its two forms:

• слабый (long adjective)
• слаб (short adjective)

Long adjectives describe a general condition

Let’s imagine a boy who is weak by nature. He’s weak in general. It’s not a temporary condition that is going to get fixed tomorrow.

In order to describe him, we’ll use a long adjective

• Это слабый мальчик. Этот мальчик слабый.

Short adjectives describe a precise moment in time

Now let’s imagine a different boy. He’s a normal person, but today he’s feeling weak because he’s ill. This boy is not weak in general, only now.

In order to describe him, we’ll use a short adjective:

• Этот мальчик слаб, потому что он болеет.

As you can see, short adjectives can never go before a noun. It’s not correct to say «слаб мальчик».

The only position for short adjectives is after the noun: мальчик слаб.