Learn Russian: Если ты посадишь здесь дерево, то оно будет расти без воды. - If you plant a tree here, then it will grow without water.

In Russian, if the condition is about the future, you must use the future tense in the Если (if) clause. English says 'If you plant...' using the present, but because the planting hasn't happened yet, Russian requires the logical future: 'If you will plant...'.

Do I always have to use то in an Если sentence?

No, it is optional. то translates to 'then' and nicely connects the two halves of the condition. You can leave it out and just use a comma, exactly like dropping 'then' in English.

Why is the future tense one word (посадишь) in the first half, but two words (будет расти) in the second half?

This is because of verb aspects. посадить is a perfective verb (a completed action: planting the tree), which forms its future tense in a single word. расти (to grow) is an imperfective verb (an ongoing process), so its future tense needs a form of быть (to be) plus the infinitive.

Why does the second part use оно instead of он or она for 'it'?

оно is the pronoun for neuter nouns. It refers back to дерево (tree), which is neuter because it ends in -о.