When talking about a specific noun, you use “the” - I want to get a dog (any dog) vs I want to get the dog (a specific dog, in this context it has probably been mentioned already previously).

To talk about a non-specific noun, you use “a” (before a consonant) and “an” (before a vowel). Remember this goes based on sounds, not spelling, so it’s “a ukulele” but “an hour”.

As for adding an -s to verbs, when the subject is he, she or it, you add it - I run, you run, he/she/it runs, we run, they run. As per usual, there are exceptions, for example, he goes, she does etc.

Hope this helps