Questions & Answers about Я показываю новый телефон подруге.
Why does the word for friend end in -е here (подруге)?
Because it is the indirect object, which is the person you are showing the phone to. This requires the Dative case. The dictionary form is подруга (female friend). Since it is a feminine noun ending in -а, the ending changes to -е in the Dative case.
Why is there no Russian word for 'to' in this sentence?
In Russian, the Dative case itself does the job of the word 'to'. Changing the ending of подруга to подруге automatically translates to 'to a friend'. You do not need to add an extra preposition.
If новый телефон is the object being acted on, why didn't its endings change?
It is actually in the Accusative case because it is the direct object (the thing being shown). However, for masculine, non-living (inanimate) nouns like телефон, the Accusative form looks exactly the same as the dictionary form. The adjective новый just matches the noun.
What if I want to show the phone to a male friend instead?
Подруга specifically means a female friend. For a male friend, the dictionary form is друг. Since it is a masculine noun ending in a consonant, its Dative case ending is -у. So, you would say другу instead.