Learn Dutch: Het stralende meisje op het portret is heel beroemd. - The radiant girl in the portrait is very famous.

It comes from the verb stralen (to radiate or to shine). In Dutch, you add -d to the full infinitive verb to make a present participle (stralend), which means 'shining' or 'radiating'. In this sentence, it is used exactly like a normal adjective to describe the girl.

Since meisje is a het-word, why does stralende get an -e at the end?

You might remember that adjectives in front of a het-word drop the -e ending. However, that only happens if they are preceded by een, geen, zo'n, or no article at all (for example: een stralend meisje). Because we are using the specific article Het here, the adjective follows the standard rule and keeps its -e ending.

Why does the Dutch sentence use op instead of in for the portrait?

In Dutch, you use the preposition op (literally 'on') for 2D images, photos, paintings, and screens. So while an English speaker sees the girl 'in' a portrait, a Dutch speaker sees her 'on' it (op het portret).

Could I use erg instead of heel to say 'very famous'?