use of definite article for describing a class?

[slim1111]

I've seen a few examples where the definite article is used for describing a class
For example

The lion is a fierce and dangerous animal.
or
The human heart is composed of three layers.

These sentences are simple enough to understand, but can they be used where using the indefinite article is sufficient? such as

"Students are not allowed to use the calculator"
Does it need to be "calculators" ?

or when telling somebody to do something like
"Please use the pen instead of the pencil"
Assuming we are not talking about a specific pen or pencil

 

[5jj]

1. "Students are not allowed to use the calculator"
Does it need to be "calculators" ?

or when telling somebody to do something like
2. "Please use the pen instead of the pencil. " Assuming we are not talking about a specific pen or pencil

1. 'a calculator' or 'calculators'. Here we are not really talking about the class of calculators, but about those calculator(s) that may be available to the students we are talking about.

2. This is a similar situation; so, 'a pen/pencil' or 'pens/pencils'.

 

[slim1111]

Thanks that's really helpful