Spoiler

---- Minecraft Crash Report ----

WARNING: coremods are present:
Contact their authors BEFORE contacting forge

// But it works on my machine.

Time: 7/3/17 12:17 AM
Description: Initializing game

java.lang.AbstractMethodError
    at net.minecraftforge.client.model.MultiModelState.apply(MultiModelState.java:67)
    at net.minecraftforge.client.model.MultiModelState.getPartState(MultiModelState.java:50)
    at net.minecraftforge.client.model.ModelLoader$WeightedRandomModel.bake(ModelLoader.java:779)
    at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:186)
    at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28)
    at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121)
    at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:512)
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377)
    at net.minecraft.client.main.Main.main(SourceFile:123)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Client thread
Stacktrace:
    at net.minecraftforge.client.model.MultiModelState.apply(MultiModelState.java:67)
    at net.minecraftforge.client.model.MultiModelState.getPartState(MultiModelState.java:50)
    at net.minecraftforge.client.model.ModelLoader$WeightedRandomModel.bake(ModelLoader.java:779)
    at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:186)
    at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28)
    at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121)
    at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:512)

-- Initialization --
Details:
Stacktrace:
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377)
    at net.minecraft.client.main.Main.main(SourceFile:123)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

-- System Details --
Details:
    Minecraft Version: 1.12
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_25, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 135132040 bytes (128 MB) / 344133632 bytes (328 MB) up to 1060372480 bytes (1011 MB)
    JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
    IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
    FML: MCP 9.40 Powered by Forge 14.21.1.2387 Optifine OptiFine_1.12_HD_U_C3 4 mods loaded, 4 mods active
    States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
    UCH    minecraft{1.12} [Minecraft] (minecraft.jar) 
    UCH    mcp{9.19} [Minecraft Coder Pack] (minecraft.jar) 
    UCH    FML{8.0.99.99} [Forge Mod Loader] (forge-1.12-14.21.1.2387.jar) 
    UCH    forge{14.21.1.2387} [Minecraft Forge] (forge-1.12-14.21.1.2387.jar) 
    Loaded coremods (and transformers): 
    GL info: ' Vendor: 'ATI Technologies Inc.' Version: '4.5.13397 Compatibility Profile Context 15.200.1055.0' Renderer: 'AMD Radeon HD 8550G + HD 8670M Dual Graphics'
    Launched Version: 1.12-forge1.12-14.21.1.2387
    LWJGL: 2.9.4
    OpenGL: AMD Radeon HD 8550G + HD 8670M Dual Graphics GL version 4.5.13397 Compatibility Profile Context 15.200.1055.0, ATI Technologies Inc.
    GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

    Using VBOs: Yes
    Is Modded: Definitely; Client brand changed to 'fml,forge'
    Type: Client (map_client.txt)
    Resource Packs: Faithful 32x32, Codecrafted 1.11+
    Current Language: English (United Kingdom)
    Profiler Position: N/A (disabled)
    CPU: 4x AMD A8-5550M APU with Radeon(tm) HD Graphics 
    OptiFine Version: OptiFine_1.12_HD_U_C3
    Render Distance Chunks: 10
    Mipmaps: 4
    Anisotropic Filtering: 1
    Antialiasing: 0
    Multitexture: false
    Shaders: null
    OpenGlVersion: 4.5.13397 Compatibility Profile Context 15.200.1055.0
    OpenGlRenderer: AMD Radeon HD 8550G + HD 8670M Dual Graphics
    OpenGlVendor: ATI Technologies Inc.
    CpuCount: 4

This happends when I put OptiFine into the mods folder...

I hope someone can help.

Optifine requires a specific Forge version, read the install instructions.