cord and ropeーwhen are they uncountable?
tie sth up: to fasten sth with cord, rope, etc

We need a longer cord for this telephone.

How do I learn which one to use, countable or uncountable?

Question

cord and ropeーwhen are they uncountable?
tie sth up: to fasten sth with cord, rope, etc

We need a longer cord for this telephone.

How do I learn which one to use, countable or uncountable?

Answer

The uncountable form refers to the material ( grammatically, the same as wood, wool or metal)
The countable form refers to a specific object - in this case, a length of cord of a particular size.