Causative Form

Causative Form
Ichidan Verbs: Verb Stem + させる
Godan Verbs: ない Stem + せる
Irregular Verbs: する→させる ・ くる→来させる

The causative form expresses causation. It will mean either:

• “Allowed something to do x” or
• “Made something do x”.

Grammatically, it functions like あげる with particles, and you could think of it as the “giving” (permission, forced work) form of a verb. 
(It’s called the ”Employment/labor” form in Japanese).

Let do

To let X do verb
X(に or を) + Causative form verb

The most basic meaning of the causative form is “let”. It is used to describe someone allowing you to do something (that you might not be allowed to do). The topic/subject of the sentence is the allower, and the allowed person is typically marked with に。See the bottom of this page for more information.

Allower は/が + Allowed person に/を + Causative Verb

Examples

• 母は私に好きなだけご飯を食べさせた – My mom let me eat as much food as I wanted. 
• 私に昼まで寝させた – Someone/something let me sleep until noon.
• 彼に飲ませた薬は苦かった。- The medicine (someone) let him drink was bitter.
  
  

Made do

To make X do verb
X(に or を) + Causative form verb

The other meaning of the causative form is “made to do”. It’s probably closer to a true “causative” meaning; as someone is the direct cause of someone else doing something. You may use に or を here.

Examples

• 子供に宿題をさせた – Made the kid do their homework
• 彼に魚をたべせた – Made him eat fish. 
• 子供を勉強させた – Made/let the kid study.

Shortened

Shortened causative:
Ichidan verbs: させる→さす
Godan verbs: ない stem +す (Should end in an あす sound)
Irregular: する→さす ・ くる→こさす

You may skip this section.
The causative form my be shortened as described above. This is somewhat rare and a bit informal and by the time you need to know this you should be able to figure it out based on how similar it is. It is more common in the past tense. 

• 食べさした – Let them eat./Made them eat.
• 帰らした – Let/made them go home. 
• 話さした – Made them talk. 

に or を？

Causative Particles
Transitive: Always に
Intransitive: に or を
Emotions: を
Non sentient objects: Always を

Whether or not to use に or を with the causative form is a little bit tricky.
Nuance wise:

• を marks what the action is done to
• に marks who or what is allowed or made to perform

Generally:

• に must be used with transitive verbs (If the forced party is marked), as the verbs will need を for the object. 
• You should never use 2 をs in the same verb clause with causative forms.
• を is used for emotion words. (怒らせる、泣かせる. etc.）
• に or を may be used for other intransitive verbs, but を stresses the forced aspect (see causative passive)
• If the thing being forced is non human/sentient, you will use を. (値段を安定させる）
• If the intransitive verb being forced is something bad (work), its even more likely to use を。

If you are having trouble, try to default to に except for emotions and non sentient things. 

Reference (helped me a lot with this section, worth a read)