Learn Russian: Длина стола — двадцать сантиметров. - The length of the table is twenty centimeters.

The dash replaces the present tense of the verb "to be" (is). In Russian, when you link a subject (like длина) directly to a quantity or another noun (like двадцать сантиметров), you often use a dash to separate them.

Why is стол changed to стола here?

Стола is in the Genitive case, which acts like the English word "of." Here, it connects two nouns, so длина стола translates literally to "the length of the table."

Why is the word for centimeters (сантиметров) in the Genitive plural?

In Russian, quantities ending in numbers 5 through 20 require the noun being counted to be in the Genitive plural. Since the number here is двадцать (20), the masculine noun сантиметр takes the Genitive plural ending -ов.

How would this sentence change if the length was 1.5 meters, using the current lesson's word полтора?

It would be: Длина стола — полтора метра. Because метр is masculine, you use полтора rather than the feminine полторы. Remember that полтора requires the noun to be in the Genitive singular (метра), unlike the number 20 which required the Genitive plural.