You may want to use mods for your Minecraft. But then, when you have downloaded and installed it, it does not load. It seems that your mod is not working. If this happens, surely it is annoying moreover if you do not know what to do with it.
Well, if you face any problem such as not loading, it may be because you download the fake version. A source in a Minecraft forum states that if you want to download it, it is better for you to do it in curseforge. Why? It is because downloading mod there is safer and more legit.
Then, you also have to think about what part are you trying to work out with the mods tab. There is the Launch Options tab where you choose Minecraft versions, you choose the Forge version or make a Profile for modded versions of the game.
If you want your mod works well, you have to install Forge or consider the Minecraft version you want your modded. If you want 1.12.2, then you have to go for 1.12.2. If you have downloaded it, you need to go to your downloads folder or download location. You must double click the Forge method. You can use .jar method. You will see a message which pops up and you have to choose Install Client and then Forge will do its thing and you are done here.
When you want to download mods, you have to make sure that you download it from safe sources such as Curse Forge site. You are also able to use Planetminecraft but it depends on the links because as you know that there are a lot of creators are able to send you to ad sites that are not safe. There are also other sites such as Minecraftmods, 9minecraft, 6minecraft and many more and those are mod repost sites and are Community Content stealers or community content file manipulators. However, Curseforge is safe and it hosts community content with no redirections for downloads to the content.
It is important to know the version of Minecraft or Forge that you installed. It is because the mods for that version only can work with that version. So, if you want 1.12.2 and a 1.12 mod exists, 1.12 will not work but 1.12.2 will work. It may be complicated for you but it is important for you to know that it is how it actually works. If you want modded 1.12.2, you need the Forge profile that you just installed before.
You have to put your mods into your mods folder. Forge 1.12.2 and before permits for version folders in your mods folder. You have to make sure that you do not mix up mods. It will get awkward in case the mod file does not state the version it is for or otherwise.
Well, hopefully this information can be helpful for you to know why your mod is not loading. You are able to come to forum about Minecraft to know about Minecraft including about Minecraft mods not loading there and you are able to discuss your problem about Minecraft mod there if you still face any problem.