Learn Icelandic: Bleiki kjóllinn er fallegur. - The pink dress is beautiful.

This shows the difference between weak and strong adjectives. Bleiki is placed right before a definite noun (kjóllinn, meaning the dress), so it takes the weak ending -i. Fallegur stands alone after the verb er (is), so it uses the strong masculine ending -ur to agree with the masculine noun kjóll.

Where is the word for 'the' in this sentence?

In Icelandic, the definite article (the word for 'the') is usually attached to the end of the noun as a suffix. The base noun here is kjóll (dress). By adding the masculine definite suffix -inn, it becomes kjóllinn (the dress).

How can I tell the gender of the noun kjóllinn just by looking at this sentence?

There are two big clues showing that it is a masculine noun. First, it uses the masculine definite suffix -inn. Second, the describing adjective fallegur uses the classic masculine nominative ending -ur to match it.

If I want to look up the word for 'pink' in the dictionary, what form should I search for?

You would look up bleikur. Dictionaries list adjectives in their strong, masculine, singular form. It changes to bleiki in this sentence because it is modifying a definite noun.