"I have seen the thermometer somewhere."
Translation:Ik heb de thermometer ergens gezien.
I guess you already know the general word order, but to make this a full explanation, I'll include it anyway. See explanation here general word order point 3.
So it's: subject (ik) + conjugated verb (heb) + rest (de thermometer ergens) + participle (gezien).
Also dutchgrammar has an extensive section about word order, at fist glance this looks very good, extensive and easy to understand. Specifically you're asking why the manner (ergens) goes after the specific direct object (de thermometer), that's explained here and the pages after (click "next page" a few times).
So to clarify the difference between a specific and a non-specific direct object:
- Ik heb de thermometer ergens gezien. (specific direct object)
- Ik heb ergens een thermometer gezien. (non-specific direct object)
Ha ha, well it looks like you got something out of this as well. I find it funny how you often don't know the rules behind your own language, simply because you don't have to—you already know how to speak it. I probably know more rules for German than English. ;)