Learn Spanish (Latin America): Escribí una reseña sobre este restaurante tan recomendable. - I wrote a review about this highly recommendable restaurant.

While muy recomendable means very recommendable, using tan translates closer to so recommendable. It adds a bit of emotional emphasis to your statement, as if saying a restaurant that I recommend so highly.

I always mix up este and esto. Why do we use este here?

You use este (this) when pointing out a specific masculine singular noun, like restaurante. You only use esto when referring to an abstract idea, a situation, or an unknown object. Because the word for 'this' is placed directly in front of the noun, it has to be este.

Is a reseña only used for restaurant reviews?

No, a reseña can be a review or critique of almost anything. You can write a reseña about a book, a movie, a gadget, or a hotel.

Why use the preterite escribí instead of the imperfect escribía?

Escribí (I wrote) is used because the action of writing this specific review is a single, completed event in the past. If you used escribía, it would change the meaning to I was writing or I used to write reviews.