It must have been Tom who parked the car like that,as he’s the only one who can drive.

定语从句充当主语该如何正确翻译，肯定是汤姆把车停成那样的，由于他是唯一会开车的人，为何反而是从主句翻译更顺一点。

定语从句能作主语吗？这是强调句型，强调主语。不强调的句式为：Tom musthave parked the car like that, ...。你的翻译基本正确，实质是根据强调句型翻译的。