Learn Swedish: Maten började tina och förlusten var enorm. - The food started to thaw and the loss was enormous.

With the verb börja (to start), the infinitive marker att is optional and frequently dropped in everyday Swedish. Both började tina and började att tina are perfectly correct.

How is the word förlusten formed?

It is the definite form of en förlust (a loss), which comes from the verb förlora (to lose). Adding -st to a verb stem is a classic way to create a noun in Swedish.

Does tina mean "to thaw" as in thawing by itself, or can I use it when I am actively thawing something?

It works for both! Just like in English, you can say that the food thaws (maten tinar) or that you thaw the food (du tinar maten). You will also often see it used with the particle upp (tina upp) to emphasize completely defrosting something.

Why is the adjective written as enorm here instead of enormt?

Adjectives need to match the gender of the noun they describe. Since förlust is an en-word (which we know because it takes the suffix -en to mean "the loss"), the adjective takes the basic form enorm. If it were describing an ett-word, it would be enormt.