Learn Hardcore German: Ich lese den Text langsam.

In German, the direct object of a verb takes the accusative case. Text is masculine (der Text in the nominative), so its accusative form is den Text. The structure is:

Ich (subject, nominative)
lese (verb)
den Text (direct object, accusative)

How can I tell that Text is masculine?

noun gender in German often has to be memorized, but there are some clues:

Many nouns ending in -t derived from Latin or English (like das Datum, der Moment, der Text) are masculine.
Dictionaries always list the article (der/die/das).
Over time you’ll build an instinct for common patterns, but initially you’ll need to learn each noun’s gender.

Why is langsam not inflected (no ending like langsame)?

Here langsam is an adverb of manner (describing how you read), not an adjective modifying a noun. Adverbs remain uninflected in German, so you simply use langsam. If you were to describe a noun you’d need an adjective ending (e.g. der langsame Leser).

Could I place langsam somewhere else in the sentence?