En retard vs. tardif
In French, both en retard and tardif translate to the English concepts of being late or tardy, but they are used in completely different grammatical contexts. The primary distinction is that en retard describes the state of being behind schedule, whereas tardif is an adjective used to directly describe a noun that occurs late in a timeline, day, or season.
En retard
The phrase en retard is used to express that a person, vehicle, or event is behind schedule, delayed, or arriving after a set expectation. It functions as an adverbial or adjective phrase and is almost always used in conjunction with verbs like être (to be) or arriver (to arrive).
Le train est en retard à cause de la forte tempête de neige.
(The train is late because of the heavy snowstorm.)
Je suis en retard pour mon rendez-vous chez le dentiste.
(I am late for my dentist appointment.)
Nous ne voulons pas arriver en retard au concert ce soir.
(We do not want to arrive late to the concert tonight.)
Son vol depuis Paris est en retard de deux heures.
(His flight from Paris is delayed by two hours.)
Elle s'est excusée d'être en retard pour la réunion d'équipe.
(She apologized for being tardy to the team meeting.)
Ne sois pas en retard pour le dîner chez tes parents.
(Do not be late for dinner at your parents' house.)
Tardif
The word tardif is an adjective used to describe a noun that happens late in a process, late in the day, or late in a season. Because it is an adjective, it must agree in gender and number with the noun it modifies, taking forms such as tardive, tardifs, or tardives.
Nous avons pris un déjeuner tardif après avoir terminé le projet.
(We had a late lunch after finishing the project.)
Son diagnostic tardif a malheureusement compliqué le traitement médical.
(His late diagnosis unfortunately complicated the medical treatment.)
C'est une variété de pomme à floraison tardive qui résiste au froid.
(It is a late-blooming apple variety that resists the cold.)
Ils ont envoyé une réponse tardive à notre invitation de mariage.
(They sent a late reply to our wedding invitation.)
Les gelées tardives de ce printemps ont ruiné la récolte de raisins.
(The late frosts this spring ruined the grape harvest.)
Le joueur a fait un retour tardif à la compétition après sa blessure.
(The player made a late return to the competition after his injury.)
Summary
To choose between these words, look at the grammar and the context of your sentence. Use en retard with verbs to state that someone or something missed a deadline, is running behind schedule, or is physically tardy. Use tardif as an adjective to directly modify a noun that is delayed in its nature or occurs late in a day, season, or overarching process.