"First Love" that looked like it was going to lead to marriage but ultimately dissolved was with an ISTJ male (never typed, but I am relatively certain). The dissolution was a mutual decision but based
more on his feeling that he was not "good enough for me" in the sense of being able to meet my needs emotionally. At the time I took this as a rejection but with time have realized that he was being vulnerable and honest. We recently met up again and had a bitter-sweet re-evaluation...
Onward to the questions that are within my scope of answering:
When it’s working – What are the joys and positive aspects of these relationships?
Both parties are respectful of individual interests and time. Often both share firm values and an off-beat humour or appreciation for such. The ISTJ can bring a practical, rational approach to the INFP and help to centre and organize an INFP's emotions (if they don't dismiss them as invalid). The INFP can introduce an element of adventure, sweetness, feeling and warmth to the life of an ISTJ.
Both can enjoy passion physically and the romance can be intense at the beginning and continue on to burn brightly.
-How compatible do you think these two types are in general?
I think a healthy and enjoyable relationship is possible if the two parties approach it in an emotionally mature way and are able to give and take and embrace differences, not despair of them.
-Why are they attracted to each other?
Honestly, I think a lot of it has to do with being so different from one another and yet sharing a spirit of understanding. ISTJ's in my experience run deep and are very expressive and loving though not publicly and more through actions than words.
-How to they compliment each other?
Through intelligence, bringing out qualities in each other that may need coaxing to be more recognized, humour, enjoying being together even if both are engaged in pursuits independently at home, an ISTJ's ability to sort an INFP, an INFP's ability to exhilarate an ISTJ and make them feel adored.
-How well do they understand each other and why?
Not very well inherently. The INFP will naturally try to understand what the ISTJ is thinking/feeling and will most likely be incorrect. The INFP needs to realize it is better to ask then assume. The ISTJ will often think the INFP is overly dramatic or requires huge amounts of emotional attention because of their own perception of the space feelings take. The INFP needs to have other sources of emotional support and not expect the ISTJ to be their "everything" (never healthy in any case!).
When it’s not working – What are the challenges when two people of this type are in a relationship?
-What are some of the communication challenges they can have?
Different ways of resolving differences. The INFP taking things personally. The ISTJ feeling that emotions are not rational and 'do not count as valid'. These can easily be overcome if the two parties appreciate each other as individuals and look at the components as things to be valued and not think "well, my way is better". Just, depends how the couple approaches it.
-What are the biggest frustrations between these two types?
I always felt that I never really knew what my ISTJ was thinking. He always felt badly that he was not communicating enough with me. I hesitated to ask more questions knowing that it annoyed him. He was stressed knowing I desired more connection and feeling unable to provide it. I think this was largely circumstantial (due to his upbringing) and hesitate to speak for ISTJ-hood in general.
-How can they take each other for granted?
As soon as you start to get busy, distracted or complacent you start to forget to be fascinated with what is important to your person. It's highly individual regardless of type. Taking your Love for granted or forgetting to keep courting them spells death (or boredom) for relationships. For ISTJ friends or lovers I would respect the time they need for solo endeavours. I would appreciate the way they show love and affection and listen carefully to their actions. For INFP's I would listen to them without being dismissive, excuse their occasional silliness, support their dreams, be a safe place to fall and be honest but loving when they need a firm hand.
Remember what made you fall for each other. If the INFP's creativity and generosity made you hot and the ISTJ's math skills and ability to discern patterns and analyze plucked your heart strings...don't get irritated when 'years in' they are showing those same traits in the relationship in other practical ways
-What happens with things “go wrong†between these two types?
I honestly don't know if there is a template. I think both types are incredibly loyal to 'their people'. I was incredibly invested despite seeing red flags. My ISTJ was increasingly anxious that he was ruining my chances for a good life and convinced me we wouldn't work.
Advice for couples – What recommendations do you have?
-What things should each type do to facilitate better communication?
Be patient and try not to take things personally or react in anger. Try to view the conversation as solving a problem as a team for the betterment of the relationship. Make a plan to have a date after the conversation so that it helps to keep things more "friendly" knowing you will have to spend time together right after.
ISTJ's, realize that feelings are valid even if they feel uncomfortable or not as important as rational thoughts. Realize that INFP's may take longer to process things and may not be able to vocalize how they feel in the moment. INFP's don't badger an ISTJ into sharing with you or nag. Ask questions rather than make assumptions. Both treat the other with respect and honour.
-What advice do you have for each of the two types?
I don't feel qualified to offer advice. I only offer my own observations based on experience.
-If you are an INFP, what advice do you have for the ISTJs?
INFP's ARE incredibly different than you but that doesn't make them less than. Give them the opportunity to bring new things to your experience and encourage them to safely express themselves. If you can be patient & let them know you would rather they just be upfront than hint..they will learn to trust you and WILL be. Wrap your arms around them and they will feel like YOU are their safe place in the world and they will do anything for you (and will rock your world in ways you can't begin to imagine). Let yourself have fun but also be honest if you need things to be a bit more predictable for your own comfort.
-If you are an ISTJ, what advice would you have for the INFPs?
Obviously I am INFP but based on my past I would say to be prepared to fly less by the seat of your pants and learn to embrace routines and schedules for the comfort level of your partner at times. To value actions more than expecting loads of affection or expressions of love and to stick up for yourself and be candid as that is what the ISTJ actually desires from you.