aspect - Using bare infinitive with verbs such as "see", "watch", etc.: Present tense or Past Tense?

I know using the bare infinitive after verbs such as hear, see, watch, etc. conveys a different meaning from using the present participle (verb+ing):

• I watched him climbing over the fence ( Climbing wasn't finished at the moment of speaking)


• I watched him climb over the fence ( Climbing was finished at the moment of speaking)

I noticed the majority of the Grammar books use past tense to explain the aforementioned difference. Personally, I think that is quite rational because the bare infinitive implies a completed or finished occurrence. This means that event belongs to the past. However, I have a problem conceiving similar sentences in the present tense. For example:

The questions is:

If the bare infinitive implies a completed action, then why should we use the present tense to talk about that event? (the event is in the past).

We can say that:

An uneasy feeling came over me as I watched him walk towards the revolving doors

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Added info 1

If you think my "climbing" examples are irrelevant, you can check Cambridge Advanced Grammar In Use, page 60, Unit 30, Section D or just check this link

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Added info 2

My question is from the logical point of view. I think the act of "seeing" is instantly. Therefore, it is not rational to say "see" a "completed process" but, "saw" a "completed process".

Answer

The difference between bare infinitive and gerund (-ing) is one of aspect, not tense. The bare infinitive looks at an action as a signal point in time whereas the gerund looks at it as a process (which is currently happening).

I watched him climbing over the fence

This means at the moment you watched him, the act of climbing was still in progress. Compared to:

I watched him climb over the fence

This view the action as a signal point without a beginning or end. This distinction is typically called the imperfective vs perfective aspect.

The bare infinitive doesn't imply a completed action, but rather a complete action without further structure or detail.

Thus,

An uneasy feeling came over me as I watched him walk towards the revolving doors

means the uneasy feeling occurred as the speaker watched the act of walking in its entirety.

For many circumstances, the differences between the bare infinitive and the gerund will be slight.