Breakdown of В телескоп можно посмотреть на все планеты.
в
in / to (direction)
на
on, at / by (transport) / to (a place) / to (in answer to)
посмотреть
to watch (perfective) / to look at (perfective)
можно
one may, it is allowed
Questions & Answers about В телескоп можно посмотреть на все планеты.
Why is все used here instead of всё?
Все is the plural form of the pronoun весь (all). It matches the plural noun планеты (planets). Since they are the object of the action (coming after the preposition на), they are in the Accusative case. For inanimate plural words, the Accusative form все looks exactly like the Nominative (dictionary) form. You would use всё if the noun was a singular neuter word, or to simply mean 'everything'.
Why does 'through a telescope' translate to в телескоп? Shouldn't we use через (through)?
In Russian, when you look through an optical device (like a telescope, microscope, or binoculars) or an opening like a window, you use the preposition в (into) followed by the Accusative case. You are literally saying 'look into a telescope'.
Why do we need the preposition на before все планеты?
The verb посмотреть (to look) requires the preposition на plus the Accusative case when you want to say you are looking 'at' something. Without на, the sentence would feel incomplete.
Where is the word for 'you' in this sentence?
There isn't one! The word можно is an impersonal word that means 'it is possible' or 'one can'. When you combine it with the infinitive verb посмотреть (to look), it makes a general statement meaning 'it is possible to look' or 'you can look'.