Learn Hardcore Russian: Слабый сигнал мешает мне общаться онлайн. - A weak signal prevents me from communicating online.

Мешать here means to interfere with / prevent someone from doing something. A very common pattern is:
мешать + кому (dative) + infinitive
So мешает мне общаться literally means (it) interferes with me to communicate → prevents me from communicating.
Examples:

Шум мешает мне спать. = Noise prevents me from sleeping.
Дождь мешает нам гулять. = Rain prevents us from walking.

Why is it мне and not меня?

Because мешать in this meaning takes the person affected in the dative case (кому?).
So:

мешает мне = prevents me
мешает ему = prevents him
мешает нам = prevents us
Using меня (accusative) would be incorrect with this construction.

Why is общаться in the infinitive? Why not a finite verb form?

After мешать, Russian typically uses an infinitive to express the action that is being hindered: мешает (кому) что делать.
You could rephrase the idea with a clause, but it becomes heavier:

Слабый сигнал мешает тому, чтобы я общался онлайн. (possible, but uncommon)
The infinitive version (мешает мне общаться) is the natural everyday choice.

What does the -ся in общаться mean?

-ся marks a reflexive/reciprocal verb form. With общаться, it usually means to communicate / to chat with people (mutually) rather than “to communicate something.”
Compare:

общаться (с кем-то) = to chat/communicate (with someone)
общать is not used in modern Russian in this meaning
If you want “to tell/communicate information,” you’d use other verbs like сообщать:
сообщать новости = to report/share news

Where is “with” in this sentence? Should there be с кем-то?

It’s optional because Russian can leave the “with whom” part implied.

…мешает мне общаться онлайн. = prevents me from communicating online (with people / in general)
If you want to specify who you’re chatting with, add с + instrumental:
…мешает мне общаться онлайн с друзьями. = prevents me from chatting online with friends

Why is the verb мешает (3rd person singular)? How does agreement work here?

The subject is Слабый сигнал (masculine singular nominative), so the verb agrees with it:

сигнал мешает (he/it prevents)
If the subject were plural, you’d use мешают:
Помехи мешают мне общаться онлайн. = Interference prevents me from communicating online.

Can I change the word order? What changes in emphasis?

Yes. Russian word order is flexible, and changes usually affect emphasis, not basic meaning. For example:

Слабый сигнал мешает мне общаться онлайн. (neutral)
Мне мешает общаться онлайн слабый сигнал. (emphasizes me being affected)
Онлайн мне мешает общаться слабый сигнал. (emphasizes online)
The neutral, most common version is the original.

Why is it онлайн without в? Can I say в интернете instead?

Онлайн is commonly used as an indeclinable adverb meaning online, so no preposition is required: общаться онлайн.
You can absolutely replace it with:

общаться в интернете = communicate on the internet
общаться в сети = communicate online (lit. “in the network”)
All are natural; онлайн is more concise and slightly more colloquial/modern.

Is слабый сигнал the best phrase? What are common alternatives?

Слабый сигнал is correct, especially for cell/Wi‑Fi signal strength. Common alternatives depending on context:

плохой сигнал = bad signal (very common, broader)
слабая связь = weak connection/reception (often about phone reception)
плохой интернет = bad internet (focuses on the internet service, not “signal”)
So you might hear: Слабая связь мешает мне… just as often.

How would I say the same idea in past/future, or with a “completed” sense?

мешать is imperfective (ongoing/general hindrance).

Present: мешает = is preventing
Past: мешал = was preventing
Future: будет мешать = will be preventing

If you mean “it prevented (once) / got in the way at a specific moment,” use perfective помешать:

Слабый сигнал помешал мне пообщаться онлайн. = A weak signal prevented me from chatting online (that time).