"I shall" is old fashioned english for the future tense. Ich soll = "I should", Ich sollte = "I should have", Ich hätte sollen = "I ought to have" although the last one is formal and informally you also say "I should have". We have to be realistic here the English in its modern everyday use is very mixed up with "sollte" und "hätte sollen" but is clear in German.
the thing is that "sollen" has a different meaning than english "should". It is more "expected to, supposed to" than what our "should" implies. sollten is used in the same way "should" is used. look it up in a dictionary .. I also hear "sollte" used often in speech to indicate future tense.
The top two comments in this reddit thread explain it in a way i would agree to as a German: https://www.reddit.com/r/German/comments/4zrymn/soll_vs_sollte/
Sollte is less certain to happen. In present tense, soll is generally used if there's some kind of command. Either a person has the instruction, or I want them to do something.
"Ich soll den Lehrer anrufen." (I have instructions to call the teacher)
"Er soll sich (bitte) beeilen!" (I want him to hurry up)
To increase confusion, there's also the description of future:
"Ich habe den Wetterbericht gelesen: Morgen soll es gutes Wetter geben" - I have read the weather forecast. The weather tomorrow is going to be fine. (We can plan our trip to the beach)
"Morgen sollte es gutes Wetter geben." - Weather tomorrow should be fine. - Here, I admit a little uncertainty, but I'm optimistic. (We can make plans, but we should have an alternative to our beach trip - "Wir sollten eine Alternative haben" btw.)
"Sie soll morgen den Chefposten übernehmen" - She is supposed to..., the intended course of events is that she...
this is why I disagree with DL's approach to subjunctive. It's a complex theme and it's just drip fed to us with no explanation.
Both solle and sollte are subjunctive and either can be used, in spoken deutsch solle is rarely used. You should not think of sollte as a "past tense" even though it is called the preterite form.
Late response :) As a german, the only context i would use "solle" in would be indirect speech, e.g. "Er sagte, ich solle aus dem Weg gehen." or maybe "Es hieß, das Haus solle nächste Woche abgerissen werden." (Word was that the house should be broken down next week.) There could be more, but I believe it is correct to use Konjunktiv II instead of K I in every case (I'm not sure!!). Sadly most people do that, or just ignore the whole wonderful world of Konjunktiv.
I got this link from someone else in some other thread http://www.dartmouth.edu/~german/Grammatik/Subjunctive/KonjunktivII.html
Not sure if I can explain it well, but when referring to the present tense (sollte can also be past...), "Ich soll" has a stronger sense of 'obligation' than "Ich sollte." Perhaps in "ich soll," someone told you to, but "ich sollte" is more like... you feel like its a better idea to.
Talking about the past, you can also use "ich sollte" to mean 'i should have'