assign keywords to sentences, is that possible

What I would suggest using, in that case, is the style highlight setting, when you set up or create a style. The highlight is a simple visual marker to draw attention to the styled text, and is especially useful if the style doesn’t really do much to the formatting of the text, if anything.

The next ingredient is to trigger the View ▸ Text Editing ▸ Hide Markup setting. Among other things (which can be fine-tuned in the Appearance: Textual Markings preference pane), this cleans up editorial markings from the editor, style highlighting among them, for a better proofreading environment.

To take this in another direction as well, it’s worth noting that while there is no formal “keyword by line” feature, there are features that provide that effective capability. The concept of tagging text in a detailed fashion is something we’ve seen a fair amount of discussion toward in the past, and it would be a shame to not dig into these methods, as you may find something that clicks and works really well for what you need. I create a small digest of links on this topic a few years back, one of which links to another digest.