Learn Dutch: Is deze kaas magerder dan die kaas? - Is this cheese leaner than that cheese?

The base adjective is mager (lean), which ends in an 'r'. In Dutch, when an adjective already ends in an 'r', we add -der instead of just -er to make the comparison (leaner). This extra 'd' makes the word much easier to pronounce.

Why do we use deze and die instead of dit and dat?

The word for cheese is a de-word (de kaas). For all de-words, you must use deze to say "this" and die to say "that". If it were a het-word, like het brood (the bread), you would use dit and dat.

Can I use als instead of dan here to mean "than"?

No, in standard written Dutch you must use dan after a comparative adjective like magerder. You only use als when two things are equal, for example: even mager als (as lean as). You might hear native speakers mistakenly say "magerder als" in casual speech, but dan is the correct word to learn.

Does mager only mean "lean" when talking about food?

When describing meat, cheese, or milk, mager means "lean" or "low-fat". However, it can also be used to describe people or animals, in which case it translates to "skinny" or "thin".