Learn Finnish: Opiskelija ajattelee paljon.

The dictionary form is ajatella (to think). It is a Type 3 verb because it ends in two consonants and a vowel (-lla). To conjugate a Type 3 verb, you drop the last two letters (in this case, -la) and add an e to make the stem. For the third-person singular form (matching "the student"), the rule is to double the final vowel, making it end in -ee.

Why does ajattelee have a double tt when the dictionary form ajatella only has one t?

Type 3 verbs use "reverse" consonant gradation! You might remember that for Type 1 verbs, the dictionary form is usually strong and the conjugated forms are weak. For Type 3 verbs, it's the opposite: the dictionary form (ajatella) is in the weak grade (one t), but the conjugated present tense forms become strong (double tt). This gives us the strong stem ajattele-.

Is the word opiskelija related to this lesson's verbs?

Yes! It is derived from the verb opiskella (to study), which is another Type 3 verb just like ajatella. The Finnish ending -ja or -jä is often added to a verb stem to describe a person who does that action. So, someone who studies is an opiskelija (student).

Why isn't there a pronoun like hän (he/she) before the verb?