Learn Hardcore Russian: Я уменьшаю яркость экрана вечером, чтобы легче засыпать. - I lower the screen brightness in the evening so that I can fall asleep more easily.

Уменьшаю is the 1st person singular present of the imperfective verb уменьшать. It suggests a habitual/repeated action: you do this (regularly) in the evenings.
If you use уменьшу (perfective уменьшить), it points to a single completed action (often with a future meaning): Я уменьшу яркость экрана вечером... = I will reduce the screen brightness in the evening (at some point), in order to....

How do I know that уменьшаю means I reduce / I am reducing?

Russian present-tense forms of imperfective verbs cover both English I reduce (habit) and I am reducing (in progress). Context decides. Here вечером strongly pushes the habitual reading: I reduce it in the evening(s)..

Why is there no word for I like я—oh wait, there is Я. Could it be omitted?

Yes, Russian can omit the subject pronoun because the verb ending already shows the person/number (-аю = 1st person singular).
So Уменьшаю яркость экрана вечером... is natural in context. Keeping Я adds emphasis/contrast (for example, I do this (not someone else)).

What case is яркость, and why?

Яркость is the direct object of уменьшать, so it’s in the accusative. For inanimate feminine nouns like яркость, accusative = nominative in form, so it looks unchanged: яркость.

Why is it яркость экрана and not something like an adjective meaning screen brightness?

Russian often uses a “noun + noun (genitive)” structure to express “X of Y”:
яркость экрана = brightness of the screen = screen brightness.
Here экрана is genitive singular of экран (masculine).

What case is экрана, and how do I form it?

Экрана is genitive singular of экран. For many masculine nouns, genitive singular is -а / -я:
экран → экрана.
It’s genitive because it depends on яркость (brightness of what?).

Why is вечером in that form, and what case is it?

Вечером is instrumental singular of вечер, used adverbially to mean in the evening.
This is a common “time expression” pattern: утром, днём, вечером, ночью.

Could I also say вечером as по вечерам or вечерами? What’s the difference?

Yes:

вечером = in the evening (often: as a typical time, but can be one evening too depending on context)
по вечерам = in the evenings (clearly habitual/repeated)
вечерами = in the evenings (also habitual; a bit more stylistic/literary in some contexts)

Why is there a comma before чтобы?

Because чтобы легче засыпать is a subordinate purpose clause. Russian normally separates subordinate clauses with a comma:
..., чтобы ... = ..., in order to ....

How does чтобы work here, and why is it followed by an infinitive?

Чтобы introduces purpose: so that / in order to.
When the subject is the same in both parts (here, я), Russian commonly uses чтобы + infinitive:
Я ... , чтобы ... засыпать = I ..., in order to fall asleep more easily.

When would I need чтобы + a past form (like чтобы я заснул) instead of the infinitive?

You typically use чтобы + past form when: 1) the subject changes, or
2) you want a more “finite clause” feel (often with explicit subject), or
3) you emphasize a specific outcome.
Example with same person but more explicit result: Я уменьшаю яркость, чтобы я быстрее заснул (less common, sounds heavier).
Example with different subject: Я уменьшаю яркость, чтобы ребёнок легче засыпал.

What does легче mean grammatically—why not легко?

Легче is the comparative of легко: more easily / easier.
Russian often prefers the comparative in this kind of purpose statement: чтобы легче засыпать = to fall asleep more easily.
чтобы легко засыпать is possible, but it’s more like to fall asleep easily (in general), not explicitly “easier than otherwise.”

Why is the verb засыпать imperfective? Could it be уснуть?

Засыпать (imperfective) focuses on the process of falling asleep and/or a repeated situation: to fall asleep (as a process), to be able to fall asleep.
Уснуть (perfective) focuses on the single result: to fall asleep (successfully, once).
In purpose clauses about making it generally easier, imperfective засыпать is very natural. If you mean to make sure I actually fall asleep (tonight), чтобы быстрее уснуть is also common.

Is the word order flexible here? Could I move вечером or чтобы...?

Yes, Russian word order is flexible, but it changes emphasis:

Я уменьшаю яркость экрана вечером, чтобы легче засыпать. (neutral)
Вечером я уменьшаю яркость экрана, чтобы легче засыпать. (emphasizes evening)
Чтобы легче засыпать, я уменьшаю яркость экрана вечером. (emphasizes the purpose)

Are there other natural verbs besides уменьшать for “turn down (brightness)”?

Yes, common alternatives:

уменьшать яркость (neutral, standard)
снижать яркость (slightly more formal)
убавлять яркость (colloquial/very common in speech)
делать яркость ниже (less idiomatic, but understandable)