Here’s a more natural version of your sentences: 1. I got suspicious when he offered a deal that seemed too good to be true. (This one is already perfect!) 2. His mom got worried when he didn’t come back, so she went out to look for him. (This one is also great as is!) 3. He was furious when he found out about the new deal. 4. I cover my mug with a small plate to keep my chamomile tea warm. (No changes needed—it’s natural.) 5. I cover my mug with a small plate to stop my chamomile tea from cooling down. 6. I cover my mug with a small plate so my chamomile tea stays warm. 7. He likes taking photos at dusk, but I prefer taking photos at dawn. 8. They say men with big beards tend to age badly. 9. If you don’t refrigerate milk on a hot day, it can spoil quickly. 10. If you don’t protect your eyes, you risk going blind earlier.

Most of these phrases are very natural already. Well done! Only two of them stand out to me, as being worth mentioning here. - They say men with prominent beards tend to go bad earlier. I think you mean to say 'bald'. If you fix this, then the sentence is fine. - He likes taking photos when it’s getting dark, but I, on the contrary, prefer taking photos when it’s getting light. This is not bad, but I recommend changing "on the contrary" to "on the other hand" as it's smoother, especially in casual speech. People do say "on the contrary" but it's usually in much shorter sentences - usually as a standalone phrase because it's such a quirky thing to say.