Learn Russian: Для этого текста нужно пользоваться хорошим словарём. - For this text, one needs to use a good dictionary.

The verb пользоваться (to use) requires the object you are using to be in the Instrumental case. Even though we translate it as a direct object in English, in Russian you are literally saying "to make use with" a dictionary.

Why does the adjective хорошим end in -им instead of -ым?

The noun it describes, словарём (dictionary), is masculine, so we need the masculine Instrumental ending. Usually, this is -ым (like in новым). However, the 7-letter spelling rule says we can never write the letter ы after ж, ч, ш, щ, г, к, or х. Since the stem of хороший ends in ш, we must use -им instead.

How do we get the form словарём from the basic word словарь?

Словарь is a masculine noun ending in a soft sign (-ь). To form the Instrumental case for masculine nouns ending in a soft sign, we replace the -ь with -ём (or -ем if the stress isn't on the ending). Because the stress falls on the ending here, it becomes словарём.

Why is it этого текста and not ?