It is really important to remember that Instagram is a social network. It is not just about sharing images and then leaving but it is about sharing images and also engaging with other people. The community aspect of the network is just as important as the images and the stories you tell.
There's a certain etiquette you'll pick up after a while using Instagram, so try to look in the comments of other people's posts so that you can pick up on their behaviour. Over time you'll learn about positively connecting with the community and not doing anything that’s going to annoy someone. Or indeed, thinking you’ve told someone you’ve mentioned them and then getting sad and disappointed when they don’t respond!
Tagging people isn't the same as mentioning them in a comment. It's an important difference to understand. Tagging shows up as a note overlayed over the image, and most people don't see it there. Mentions are in the comments that you put under an image, or in the caption when you first post it. Here are the two Instagram help pages that will help you if you're not clear on the difference: how to tag someone & how to mention someone.
When you want to mention someone in your image caption you can include their username but if you want to make sure that they see the image you will also need to tag them. I know lots of people mention me and I just don’t see the comments because I have a lot of notifications each day. That is such a shame for them and for me. Lots of people don’t know there is a tag they can click on, so it’s good practise to mention the person in the caption and tag them too.
If you are reposting an image that someone else has taken place don’t just tag them. This happens to me so much. People take my images, tag me but then don’t mention me in the caption. Hardly anyone will click or even see the tag, so they will think this person has taken the image. This doesn’t make me happy and if someone does it too many times then I will block them.
Think carefully before reposting someone else’s image. Think about why it might be a good thing but if you do share it make sure you are clear whose image it is, so people can follow that person too.