Countable and Uncountable

COUNTABLE.  What you  CAN count.  One, two, three, four...

One - singular.  

One table, one armchair, one rocking chair, one sofa.

A table, an armchair, a rocking chair, a sofa.

Two or more - plural.    Two chairs.  Three sofas.

UNCOUNTABLE.  What you CANNOT count.  There's no "one", no two, no plural ending.

Furniture = all of those items together, all the pieces of furniture

That is nice furniture. Is that expensive furniture? Was it expensive? That furniture is for dolls.

Furniture = kalustoa, huonekaluja

A piece of furniture  =  huonekalu