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1/ I didnt see it.

If there is a force, no need to tag the runner. If no force, then a tag is required.

At first base, the hitter running is always in a force, hence only have to touch the base with the foot.

2/ On the bases, there is a force only if the runner behind is forced by the hitter. In other words, first and second, then any play means a force is in play.