"The food is very tasty."

Translation: Tha am biadh glè bhlasta.

[ChaosMackenzie]

When do I use "blasta" and when do I use "bhlasta" ?

[silmeth]

Changing the first consonant of a word (marked in writing with added h) is called lenition and might be caused by many things in Gaelic. Here it is the word glè very that causes lenition of the following adjective.

It is explained in the tips and notes, and every case of grammatical lenition is explained in the notes to the skill where it first happens.

You can read the notes in the web browser version of Duolingo at https://duolingo.com and also on the https://duome.eu/tips/en/gd website. Unfortunately the tips and notes might not be available in the Duolingo mobile app, so if you’re on mobile, you might want to open your web browser for reading.

[Rraine7]

Lenition is very complicated, but learn gaelic has a really good section on it.

https://learngaelic.scot/grammar_hacks/lenition.jsp

In this particular case with Blasta, you mostly do it when you use common modifiers like fior (true), glè (very), ro (too), and sàr (great).

Tha cearc blasta
Tha cearc glè bhlasta