Ta-form doesn't always indicate past tense.

Long time no see, everyone! I have been busy and lazy this year, but I finally wrote one English post on a quiet day in August. I'm happy to do it. 

Let’s focus on “ta-form” today.

What expressions are used with ta-form?

              Ta toki, ta koto ga aru, ta bakari, ta tokoro, ta mama, ta ra, and so on.

Basically, a verb in ta-form means that the action of the verb is completed, but it doesn’t always indicate past tense. Here are the some examples:

1. maiasa kaisha ni tsuita (A) ato de, asagohan o taberu (B). (Every morning, I have breakfast after I arrive at my office.)
2. kino asagohan o tabeta (A) ato de, gym ni itta (B). (Yesterday, I went to the gym after I had breakfast.)
   

When action A (“kaisha ni tsuku” or “asagohan o taberu”) is completed, action B (“asagohan o taberu” or “gym ni iku”) is done. 

Chronological order:

1.  kaisha ni tsuku → asagohan o taberu 
2. asagohan o taberu → gym ni iku

Verb B changes its tense depending on context, but verb A always takes ta-form because it has a sense of completion. Particularly, even though the actions of sentence 1 are not in the past, “tsuita” (ta-form) is required.

The same rule applies to “ta toki”.

Watashi wa itsumo tsukareta toki, massage ni iku. (I always go get a massage when I get tired.)

The whole sentence is in the present, but “tsukareta” is the ta-form. This expresses that I go get a massage after getting tired.

Tsukareru → massage ni iku

Given the above chronological order, you must indicate the completion of “becoming tired”.

This works for “ta ra” in the same way.

Soko o migi ni magattara, eki ga aru. (If you turn right there, there is a station.)

Ame ga futtara, ie de eiga o miyou. (If it rains, let’s watch a movie at home.)

Kyoto ni ittara, kireina niwa ga mitai. (When I go to Kyoto, I want to see a pretty garden.)

Yamada san ga kitara, Tanaka san wa kaeru desho. (If Yamada comes, Tanaka will go home.)

Takarakuji ni atattara, nani o kaimasu ka? (If you win the lottery, what would you buy?)

Eki ni tsuitara, anata ni denwa o shimasu. (When I get to the station, I will call you.)

The first verbs (A) are all ta-form. After those actions are done, the second verbs (B) are conducted or occur. In other words, verbs B follow verbs A in terms of time.
Many people wonder why if-sentences require ta-form. I hope this explains why.