Learn Hardcore Russian: Сегодня сигнал слабее, чем вчера, поэтому мне трудно общаться онлайн. - Today the signal is weaker than yesterday, so it is hard for me to communicate online.

Russian often omits the present-tense form of to be. So Сегодня сигнал слабее literally reads Today the signal (is) weaker. Adding есть would usually sound unnatural here.

What part of speech is слабее, and why not слабый?

Слабее is the comparative form of the adjective слабый (weak).

слабый = weak (basic description)
слабее = weaker (comparison)
Because the sentence compares today with yesterday, Russian uses the comparative.

Why do we use чем in слабее, чем вчера?

In Russian, one common way to express comparisons is:

(comparative) + чем + (what you compare to)
So слабее, чем вчера = weaker than yesterday.
(Another option exists: вчерашнего after certain comparatives, but чем + вчера is the most straightforward and very common.)

Is вчера a noun in some case here?

No—вчера is an adverb meaning yesterday. It doesn’t change form and isn’t declined. It simply answers when?

Why is there a comma before чем?

A comma is typically used in comparisons like X ..., чем Y when it’s a full comparative construction. So слабее, чем вчера normally takes a comma.

What does поэтому do, and how is it different from потому что?

поэтому = therefore / so (it introduces the result)
потому что = because (it introduces the cause)

This sentence is structured as: cause → поэтому → result
Today the signal is weaker than yesterday, therefore it’s hard for me…

Why is мне in the dative case in мне трудно?

Russian often uses an impersonal pattern for feelings/difficulty:

мне трудно = it is difficult for me (literally to me it’s difficult)

So мне is dative because the difficulty is “directed” at/experienced by the person.

What is the grammar of трудно общаться?

трудно is a predicative adverb (a “state word”) meaning it’s difficult, and it commonly combines with an infinitive:

трудно + infinitive = it’s hard to do something
So трудно общаться = it’s hard to communicate/chat.

Why is общаться reflexive, and what nuance does it add?

общаться is a reflexive verb meaning to communicate / to socialize / to be in contact (with others). It often implies two-way interaction (talking with people), not just “transmitting information.” In this context it fits “chatting/communicating online.”

Why is онлайн not declined or put into a case?

онлайн is commonly used in Russian as an indeclinable adverb meaning online. It functions like вчера or сейчас: it describes how/where the communication happens, and it typically doesn’t change form.

Is the word order fixed? Could we say Мне трудно общаться онлайн, потому что сегодня сигнал слабее, чем вчера?

Yes, word order is flexible. Your version is also correct; it just changes information flow:

Original: cause first → result second (with поэтому)
Your version: result first → cause second (with потому что)

Both are natural; the choice depends on what you want to emphasize first.