Why does 'clothes' sound like 'close' when native speakers say it?

Because the voiced th in clothes sits inside a cluster that is hard to finish at speed. The careful form keeps the th and the z; fast speech drops the th and keeps the z, and the result matches the verb close. The word is still understood as clothing from the sentence, so this is a reduction, not an error. When you let the th lighten, keep the z, because without it clothes turns into the adjective close, which ends in a hiss instead of a buzz.

The shortening is not limited to one word. The same thing happens in other words that end with th plus a hissing sound: the plural of month often loses the th, the plural of cloth follows the same route, and the third-person form of breathe keeps the buzz but softens the th. The pattern is always the same, because the th is the hardest part of the cluster and the first part to go. For listening practice, use the ending as the test rather than the middle: the verb close ends in a buzz, the adjective close ends in a hiss, and careful clothes ends in a buzz followed by a hiss. If you can hear which of those three endings you produced, you can also hear which one a recording gave you.

The Two 'th' Sounds in English: Tongue, Word Lists and a Self-Test →