On a text based forum, there isn't anyway to understand the "tone" in which someone is writing. Using emojiis may help a little, but can come off as sarcastic itself. My writing style was always straightforward but I smoothed it out. I learned more diplomacy. But many people don’t – or maybe they just realise they’re firing away and leaving people feeling like they’ve been slapped in the face. Let's look at some examples:
- Stop using that and use Assimil. (Is that an order, Captain?)
- You must study longer. (Errrr, please?)
- I don't like Assimil, use FSI. (Sir! Yes sir!)
Imagine how much better these posts would sound with a bit of friendliness tossed in to smooth out that straightforward tone:
- I think that if you were to stop using that and use Assimil instead you might see faster progress? (Really? Hummm, I might try that out.)
- Would it be possible to for you to study longer? I think it would be better for you. (Maybe.)
- I don't like Assimil personally and I had better results using FSI. Perhaps you could try that too? (Sounds reasonable)
It didn’t take much to rephrase these sentences and write them with conscious attention to tone. People cannot guess your mood, or the tone in which you intended to write. So try to imagine how your post will be read, then change accordingly if you think it sounds harsh, or abrupt. Read your post as if someone wrote it. How would it make you feel? Would you feel annoyed, saddened, or self-critical? If so, then you need to reword the post to soften the language and tone. A little bit of empathy can go a long way in avoiding being too blunt. Think about how you might feel if someone were being too blunt in their conversation with you.
In addition, people from different cultures speak and write differently. Miscommunication can easily occur because of cultural differences, especially in the written form. It might not be they are being harsh, simply that it is a standard of their culture.
You know you're not always right, and what you say is rarely an absolute fact. There are perspective, conditions, opinions, other possibilities to consider. When you post, keep this in mind before setting out you're views as dogma. If you’re in a bad mood, say so. People can forgive some bluntness when they know it is not directed at them.
For the readers of posts which might "get your back up" as they say, remember that some people may be neurodivergent, e.g. have some mental divergences such as autism, Aspergers, etc. and the bluntness is a consequence of this. Cultural differences can play a large part. When I moved to the UK, being a boisterous, friendly American to a British person can mean you're a loud, annoying git who keeps bothering people. The difference between a positive-face culture and a negative-face culture. Here on the forum we're a diverse bunch, so please make allowances for the occasional slippage and try to sympathise with the poster.
Having said all of that, there really isn't any excuse for bad behaviour and you've always got the option to report to a moderator. I'll leave you with a lovely Irish proverb I learned in Dublin.
Manners cost nothing.
but it saves you as a reader a lot of head- and heartache 
It works great on people who are trying to be snarky at you 
