A lot of cognates
With cognates between English-Spanish, mainly I've only seen lists but I found this, which has explanations. These are usually the case but not always.
Words that end in –ous change to –oso
Curious Curioso Delicious Delicioso Glorious Glorioso Mysterious Misterioso Numerous Numeroso Precious Precioso Religious Religioso Tedious Tedioso
Spanish Cognate Rule 2 Words that end in –al are usually the same in English
Hospital Hospital Medical Medical Animal Animal Casual Casual Digital Digital Final Final Ideal Ideal Tropical Tropical
Spanish Cognate Rule 3 Words that end in –ct change to –cto
Act Acto Exact Exacto Correct Correcto Insect Insecto Dialect Dialecto Perfect Perfecto Product Producto Conflict Conflicto
Spanish Cognate Rule 4 Words that end in –ance change to –ancia
Ambulance Ambulancia France Francia Distance Distancia Tolerance Tolerancia Elegance Elegancia Importance Importancia Abundance Abundancia
Spanish Cognate Rule 5 Words that end in –ic change to –ico
These words are usually adjectives and have the stress (and therefore an accent) on the last syllable before the -ico
Alcoholic Alcohólico Basic Básico Fantastic Fantástico Generic Genérico Medic Médico Organic Orgánico Panic Pánico Romantic Romántico
Spanish Cognate Rule 6 Words that end in –ar are usually the same in English
Familiar Familiar Lunar Lunar Muscular Muscular Regular Regular Solar Solar Vulgar Vulgar Similar Similar Popular Popular
Spanish Cognate Rule 7 Words that end in –ary change to –ario
Canary Canario Diary Diario Glossary Glosario Estuary Estuario Primary Primario Salary Salario Vocabulary Vocabulario Temporary Temporario
Spanish Cognate Rule 8 Words that end in –ant change to –ante
Abundant Abundante Distant Distante Elegant Elegante Important Importante Mutant Mutante Vacant Vacante Tolerant Tolerante Instant Instante
Spanish Cognate Rule 9 Words that end in –ble are usually the same in English
Audible Audible Cable Cable Terrible Terrible Destructible Destructible Horrible Horrible Flexible Flexible Impossible Imposible Visible Visible
Spanish Cognate Rule 10 Words that end in –ence change to –encia
Coincidence Coincidencia Competence Competencia Difference Diferencia Excellence Excelencia Frequence Frecuencia Intelligence Inteligencia Presence Presencia Violence Violencia
Spanish Cognate Rule 11 Words that end in –id change to –ido
Note: many words following this rule have an accent on the first vowel.
Acid Ácido Fluid Fluido Rapid Rápido Solid Sólido Valid Válido Stupid Estúpido Liquid Líquido Vivid Vívido
Spanish Cognate Rule 12 Words that end in –ment change to –mento
Note: some words such as departamento may have other minor spelling changes.
Argument Argumento Cement Cemento Department Departamento Document Documento Element Elemento Ligament Ligamento Moment Momento Pigment Pigmento
((Also a helpful tip from LanguageTransfer, -tion words in English become -ción, then drop ción and add -ar (usually) for the infinitive, which can then be conjugated))
Another rule: English words that end in -ity turn into Spanish words that end in -dad. These are feminine nouns in Spanish. There may be some minor spelling changes:
City Ciudad Opportunity Oportunidad Verity (truth) Verdad Humility Humildad Capacity Capacidad Vicinity Vecindad Tranquility Tranquilidad Humidity Humedad Mobility Movilidad Utility Utilidad
Great list! I've noticed these too, but thanks for compiling them!
p.s. You could probably combine rules 4 and 10 to just "words ending with 'nce'".