Learn Finnish: He hyökkäävät täältä! - They are attacking from here!

This is the three-way directional paradigm in action! Finnish spatial adverbs change depending on movement. Täällä means you are statically 'here', and tänne means movement 'to here'. Because the attack is launching from this location, we use täältä ('from here').

What is the difference between täältä and tästä if they both mean 'from here'?

Tästä comes from the pronoun tämä (this) and usually refers to a very specific, narrow point. Täältä refers to a broader area or general vicinity. In a battle context, they are attacking from this general area, so täältä is the natural choice.

What is the base form of hyökkäävät, and why did the spelling change so much?

The dictionary form is hyökätä (to attack). This is a Type 4 verb. When conjugating Type 4 verbs, you apply reverse consonant gradation (a single k strengthens to a double kk) and the final vowel doubles. So, the stem becomes hyökkää-, and then you add the 'they' ending -vät.

Does Finnish distinguish between 'they attack' and 'they are attacking'?

No, Finnish doesn't have a separate continuous tense like English does. He hyökkäävät can mean a habitual action ('They attack') or something happening right this second ('They are attacking'). You just rely on the context of the situation!