Learn Danish: Digtet er vanskeligt at forstå. - The poem is difficult to understand.

Adjectives in Danish must agree with the noun they describe, even when separated by a verb like er. Because digtet is a neuter noun (coming from et digt), the base adjective vanskelig takes a -t at the end. If the subject were a common-gender noun like bogen (the book), you would drop the -t and say Bogen er vanskelig at forstå.

Can I use svært instead of vanskeligt in this sentence?

Yes, absolutely. Svært and vanskeligt both mean 'difficult' and are often interchangeable. Vanskeligt tends to sound a little more formal or literary, which fits nicely when talking about a poem, while svært is the most common choice in everyday conversation.

How can I tell from this sentence whether the word for poem is an en or et word?

You can tell by looking at the ending of digtet (the poem). In Danish, the definite article ('the') is attached as a suffix to the noun. Because the word ends in -et, you immediately know the base dictionary form is et digt.