Reading a scheme

Start with the conditions. A strong base, an acid, a metal, a peroxide: the reagent line narrows the reaction type before you look at a single bond.
Find what changed. Compare starting material and product atom by atom: which bond formed, which left, and whether the carbon count grew.
Substitution vs. elimination vs. addition is a bookkeeping question: same number of σ bonds, fewer, or more.
Mechanism follows structure. A tertiary or benzylic center with a weak nucleophile points to a cation; a primary center with a strong nucleophile does not.
Every example here is a real reaction with a real citation. The yields are the authors’ own.

Cyclopropanation of alkenes with carbenes

9 real reactions from the literature, on six-membered cycloalkenes, trisubstituted alkenes, enol ethers and eight-membered cycloalkenes. Each scheme shows starting materials, conditions and products, with the yield and reference where the source reports them.

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Carbene cyclopropanation: 3-methyl-2-buten-1-OL (a trisubstituted alkene) with CH2I2; Et2Zn gives (2,2-dimethylcyclopropyl)methanol (the cyclopropane).

Carbene cyclopropanationJ. Org. Chem. 2019, 84, 3780
Carbene cyclopropanation: 3,4-dihydro-2H-pyran (an enol ether) with Cl3CCO2Et, NaOMe; pentane (dichlorocarbene from the trichloroacetate) gives 7,7-dichloro-2-oxabicyclo[4.1.0]heptane (the cyclopropane).

Carbene cyclopropanationOrg. Synth. Coll. Vol. 5, p. 874
Carbene cyclopropanation: cyclooctene (an eight-membered cycloalkene) with CH3Li; ether, −35 °C gives 9,9-dibromobicyclo[6.1.0]nonane (the cyclopropane).

Carbene cyclopropanationOrg. Synth. Coll. Vol. 5, p. 306
Carbene cyclopropanation: tert-butyl 1,2,3,4-tetrahydropyridine-1-carboxylate (an enamine) with CARBENE; 180 ºC gives the cyclopropane.

Carbene cyclopropanationRealOChem worksheet
Carbene cyclopropanation: a 1,1-disubstituted alkene with Zn/Cu; CH2I2 gives the cyclopropane.

Carbene cyclopropanationRealOChem worksheet
Carbene cyclopropanation: 2,3-dimethyl-2-butene (a tetrasubstituted alkene) with CHBr3; tBuOK gives 1,1-dibromo-2,2,3,3-tetramethylcyclopropane.

Carbene cyclopropanationRealOChem worksheet
Carbene cyclopropanation: a conjugated diene with CH2Cl2; Et2Zn; (1 equiv) gives the cyclopropane.

Carbene cyclopropanationRealOChem worksheet
Carbene cyclopropanation: cyclohexene (a six-membered cycloalkene) with CH2I2, Zn(Cu); ether, Δ gives bicyclo(4.1.0)heptane (the cyclopropane).

Carbene cyclopropanationOrg. Synth. Coll. Vol. 5, p. 855
Carbene cyclopropanation: 1,4-cyclohexadiene (a six-membered cycloalkene) with t-BuOK; CHCl3, 0-5 °C gives 7,7-dichlorobicyclo[4.1.0]hept-3-ene (the cyclopropane).

Carbene cyclopropanationOrg. Synth. Coll. Vol. 6, p. 87