Today, whilen I was working, I spilled coffee on my desk and onto myself.

I didn't close my tumbler's lid properly, so when I tried to drink, coffee camespilled out.

If you write 'I didn't close my tumbler's lid', it sounds like the lid was not on the tumbler from the beginning (外した). But because you wrote きちんと閉められなくて am I right to assume the coffee spilled out between the lid and the tumbler ?

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I washed the coffee spot, sotain off, but the part of the shirt around my belly got soaked.

I thought it would dry soon, but it hadidn't dried yet wheny by the time I got off work and it wasso I felt cold.

I sSpilled cCoffee

I didn't close my tumbler's lid, so wlid. When I tried to drink, coffee came out of the lid.

I wearore a dark green long -sleeved shirt, so luckily, the stain doesn't stand out.

I wearore a dark green long -sleeve shirt, so luckily, the stain doesn't stand out.

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I washed the coffee spot, so the shirt around my belly got soaked.

"belly" is a little less polite, and tend to be used for people with chubbier tummies (eg. "beer belly"). You can say "stomach" in this context. In case you're wondering, "tummy" also refers to stomachs but tend to be used in a gentler context, for example, when talking to/about children.

I wearas wearing a dark green long -sleeve shirt, so luckily, the stain doesidn't stand out.

I thought it would dry soon, but it hadn't dried yet when I got off work and, so it was cold.

I wearore a dark green long -sleeve shirt, so luckily, the stain doesn't stand out.

I thought it would dry soon, but it hadn't dried ywas still wet when I got off work and it was cold.

Today, whilen I was working, I spilled coffee onall over my desk and myself.

I didn't close mythe tumbler's lid, so when I triedlifted it to drink, coffee came out.

I'm wearing a dark green long -sleeve shirt, so luckily, the stain doesn't stand out.

Or: "... so luckily, you don't really see the stain."

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I washed the coffee spottain, so the shirt around my bellystomach got soaked.

I thought it would dry soonfaster, but it hadn't dried yet whenby the time I got off work, and it was cold outside.