Solution:
- Your clothes are in the bathroom. (Clothes is a plural noun).
- Where is your other trainer? I can't find it. (Trainer is singular, so we use 'is'. 'it/them' refers to the singular trainer, so we use 'it').
- The pyjamas are too big for her. (Pyjamas is a plural noun).
- There is a white trainer under the bed. (Trainer is singular, so we use 'is').
- Jeans are always in fashion. (Jeans is a plural noun).
Answer: 1. are, 2. is, it, 3. are, 4. is, 5. are