"I am eating butter."
Translation:Je mange du beurre.
"le" translates "the": I am eating the butter = je mange le beurre.
"du" is the contraction of "de-le" and it is partitive, which means that it represents a part (a piece, a certain quantity) of something that you cannot count.
"I am eating butter" = I am eating some butter, a little butter, etc... => je mange du beurre.
Another sentence "I like eating cheese" is translated with "J'aime manger du fromage" and "J'aime manger le fromage" is considered to be wrong. Why? Is it because you generally speak of the consumption of food whereas here ("I am eating butter") you specify that you are eating "some" butter? I'm really confused with this!
"j'aime le fromage, j'adore le beurre, je préfère les confitures, je déteste la soupe... " all these sentences use definite article "le, la, les" because the verbs I used are appreciation verbs, which naturally introduce generalities.
Generalities in French are built with definite article le, la or les, whichever the noun that follows (countable or uncountable objects).
With action verbs (manger, boire, respirer, prendre...) + an uncountable, singular noun, you have to use the partitive construction (see above comment).
I like eating cheese, butter or soup = j'aime manger du fromage, du beurre ou de la soupe (some of each, an undefined quantity of each of those substances).
I like eating the butter I bought = j'aime manger le beurre que j'ai acheté: when the English use "the", you know that the substance is defined (here by "i bought/que j'ai acheté). Then, the French also use the definite article.