“You can substitute plain yogurt for the sour cream if you want to make it lower-fat.”

You're at a potluck lunch. Someone asks you about the dish that you made, so you're telling her about the recipe. You're talking about different options for the recipe, and you say:

You can substitute plain yogurt for the sour cream if you want to make it lower-fat.

substitute (something) for (something)

In cooking, to "substitute ___ for ___" means to use a different ingredient than the recipe suggests:

When I make it, I substitute ground turkey for the beef that the recipe calls for.

plain yogurt

"Plain" is used with different food products to mean that there's no extra flavor added to it. Products that you can buy "plain" include:

• plain oatmeal
• plain rice
• plain M&Ms (I don't know if M&Ms are available everywhere, but it's a chocolate candy with a hard shell on the outside. The "plain" ones don't have peanuts inside.

You should also learn the phrase "eat (something) plain", which means that you eat a simple type of food without adding anything to it:

I don't put butter or anything on my toast. I just eat it plain.

sour cream

Sour cream is a kind of dairy food that tastes a little sour. It's used in a lot of dips, and often eaten with Mexican food.

lower-fat

Food that doesn't have a lot of fat calories is called "low-fat food". People who want to lose weight try to eat low-fat food. You can use "low-fat" in a couple of ways:

Is it low-fat?

I don't like the taste of low-fat food.

"Low-fat" means that an item doesn't have much fat at all. If you're making something, like cookies, that usually have a lot of fat, but your version has less, you call it "lower-fat"