Biékàn tā mǎnkǒu-dāyìng, néng bu néng zuòdào háishì liǎng huí shì.
Don't be fooled by his ready agreement - whether he can actually deliver is another matter.
Tips
usage
Often carries a slightly sceptical tone - the speaker hints the promise may not be kept. 满口 ('mouth-full') suggests the agreement is easy to say but not necessarily easy to follow through on.
memory
Picture someone nodding with their 口 (mouth) full of yeses - the answer comes out before thought does.
